Refinishing: Who has tried a shortcut?

John Galloway wrote:

With respect, you cannot establish good practice by
totting up numbers of for and against contributions
to RAS.

No, but by spending a few hours trolling RAS, I can avoid making an
expensive mistake.

There has been quite a lot of feedback on this topic, and I have also
received a number of private responses by e-mail.

Clearly the situation is not that simple. But from the feedback I think
I can draw a few conclusions:

- The old gel coat should be removed, based on condition, not on
principal. This is a tough call, as no two people will have the same
assessment of any glider. Definitely all loose, flaking or damaged gel
coat must come off. The tough call is how deep does one go to remove
micro cracks. If you go down to glass you have extra work and/or
complications applying the new coating onto the glass surface and
restoring the profile.

- There is some debate over whether or not to use power tools. It seems
JJ's "air file" is the weapon of choice for best results and least
damage to the structure. With power tools it seems possible to remove
close to all of the gel coat if you need to. See Ken's website
referenced below for a good description of the "air file".

- There is some debate as to to the choice of filler to replace the
removed gel coat. Choices are polyester based (gel coat or filler) or
polyurathane based primers.

- I am a still unsure of what is required to restore a proper
aerodynamic profile. The original gel coat gets its shape from the
mould. Fibre and resin is laminated on top to form the structure. How
uniform is the thickness of the factory gel coat? If you remove all or
most of it, then replace it with a layer of filler and sand that filler
to a smooth contour, will the resulting shape match the original
profile close enough to avoid a significant aerodynamic penalty? How
much deviation from the airfoil shape can be tolerated before a
noticeable loss in performance occurs? The commercial shops don't seem
to labour this point. Is it necessary to check the profile with profile
gauges? If so, how far back from the leading edge should one measure?

- For refinishing, polyurathane seems to have clear advantages over gel
coat. There are no obvious negatives to it either. However gel coat is
the choice of purists and it can also be blended into the existing
factory finish for a partial refinish. Gel coat refinishes seem more
susceptible premature failure than polyurathane ones.

- Reapplication is not without problems. Blow holes, silicon
contamination etc cause problems which add time/money to the project.
The other universal problem is sanding through the new coating while
attempting to restore the contour or to remove an imperfection.

- There are a couple of excellent articles by Ken Kochanski on the net:

http://sailplane-racing.org/Articles...asw20_fuse.htm

http://sailplane-racing.org/Articles...asw20_wing.htm

These should be compulsory reading for all of us who own sailplanes with
typical 10 year old finishes. Thanks Ken. Ken used gel coat for his
project but the articles give good incite. Even the writeup on
replacing internal control seals is worth reading.

- It takes at least 300 hours to refinish an entire glider. Add more
time for repairs and modifications etc.

- A good job is one where the mass of material added is no more than the
mass of material removed. However I doubt anybody ever achieves this
objective.

- The factory finish on many gliders delivered over the last 25 years is
a weak point. Many aircraft need re-finishing. While others (Kestrel
19m and Grob have been sighted) seem to last better. Clearly the
factory finishes have evolved around streamlining the manufacturing
process and improving the look of the delivered product. Less attention
has been paid to the longevity of the product. The suggestion that
sanding marks from the factory finish are actually the cause of gel
coat cracks is very plausible. It seems like a Good Idea to get a
polyurathane coat over the gel coat, sooner rather than later. Even DG
have come to this conclusion:

http://www.dg-flugzeugbau.com/pur-lack-e.htm

Thanks to all those who have contributed, both in RAS and in private
e-mail.


Ian

I have stayed out of the debate so far, because I have very
limited experience. I have done three gliders, one of them
a complete refinish, a Cirrus. The Cirrus was done almost
ten years ago and the finish is still like new. The crazing
was extensive . The gel coat was taken off, to or near the
point of making contact with the glas. I maintained the airfoil
shape through out the process, which caused the old gel
coat to be thicker in some areas of the wing than others.
The cracks by then were so fine, they were barely visible.
After that, several stages of reapplying gel coats were done
with sandings in-between. The key was to build and sand till no
more marking of the crazing was visible.

As for maintaining the correct shape of the airfoil, I have a
rule of thumb that it should not exceed 5% overall, but
maintain a very close tolerance when it comes to waviness.
See Dick Johnson's articles on what makes laminar flow.

I have used Duratec and a high quality gel coat, mixed at
1:5 ratio, which gives it a paint like application and finish,
which keeps the orange peel to a minimum.
I know of at least three other applications with Duratec
and the results are still good. I am convinced that the
product, because it was designed for refinishing high temp
moulds, produces a more durable result.

On later projects I used epoxy sanding primer and Urethane.
It is a bit more difficult to sand the finish coat, but the results
are worth it.
Regards
Udo






"Ian Forbes" wrote in message
...
John Galloway wrote:

With respect, you cannot establish good practice by
totting up numbers of for and against contributions
to RAS.

No, but by spending a few hours trolling RAS, I can avoid making an
expensive mistake.

There has been quite a lot of feedback on this topic, and I have also
received a number of private responses by e-mail.

Clearly the situation is not that simple. But from the feedback I think
I can draw a few conclusions:

- The old gel coat should be removed, based on condition, not on
principal. This is a tough call, as no two people will have the same
assessment of any glider. Definitely all loose, flaking or damaged gel
coat must come off. The tough call is how deep does one go to remove
micro cracks. If you go down to glass you have extra work and/or
complications applying the new coating onto the glass surface and
restoring the profile.

- There is some debate over whether or not to use power tools. It seems
JJ's "air file" is the weapon of choice for best results and least
damage to the structure. With power tools it seems possible to remove
close to all of the gel coat if you need to. See Ken's website
referenced below for a good description of the "air file".

- There is some debate as to to the choice of filler to replace the
removed gel coat. Choices are polyester based (gel coat or filler) or
polyurathane based primers.

- I am a still unsure of what is required to restore a proper
aerodynamic profile. The original gel coat gets its shape from the
mould. Fibre and resin is laminated on top to form the structure. How
uniform is the thickness of the factory gel coat? If you remove all or
most of it, then replace it with a layer of filler and sand that filler
to a smooth contour, will the resulting shape match the original
profile close enough to avoid a significant aerodynamic penalty? How
much deviation from the airfoil shape can be tolerated before a
noticeable loss in performance occurs? The commercial shops don't seem
to labour this point. Is it necessary to check the profile with profile
gauges? If so, how far back from the leading edge should one measure?

- For refinishing, polyurathane seems to have clear advantages over gel
coat. There are no obvious negatives to it either. However gel coat is
the choice of purists and it can also be blended into the existing
factory finish for a partial refinish. Gel coat refinishes seem more
susceptible premature failure than polyurathane ones.

- Reapplication is not without problems. Blow holes, silicon
contamination etc cause problems which add time/money to the project.
The other universal problem is sanding through the new coating while
attempting to restore the contour or to remove an imperfection.

- There are a couple of excellent articles by Ken Kochanski on the net:

http://sailplane-racing.org/Articles...asw20_fuse.htm

http://sailplane-racing.org/Articles...asw20_wing.htm

These should be compulsory reading for all of us who own sailplanes with
typical 10 year old finishes. Thanks Ken. Ken used gel coat for his
project but the articles give good incite. Even the writeup on
replacing internal control seals is worth reading.

- It takes at least 300 hours to refinish an entire glider. Add more
time for repairs and modifications etc.

- A good job is one where the mass of material added is no more than the
mass of material removed. However I doubt anybody ever achieves this
objective.

- The factory finish on many gliders delivered over the last 25 years is
a weak point. Many aircraft need re-finishing. While others (Kestrel
19m and Grob have been sighted) seem to last better. Clearly the
factory finishes have evolved around streamlining the manufacturing
process and improving the look of the delivered product. Less attention
has been paid to the longevity of the product. The suggestion that
sanding marks from the factory finish are actually the cause of gel
coat cracks is very plausible. It seems like a Good Idea to get a
polyurathane coat over the gel coat, sooner rather than later. Even DG
have come to this conclusion:

http://www.dg-flugzeugbau.com/pur-lack-e.htm

Thanks to all those who have contributed, both in RAS and in private
e-mail.


Ian

On this thread there have been several references to the lack of problems
with the gelcoat on the Kestrel 19.

The two Kestrels that I owned had no gelcoat cracking up to ten years after
manufacture, although one of them developed shrinkage along the spar line.

In Martin Simons book on Slingsby sailplanes he makes reference to the fact
that George Burton, the managing director, was unable to get the wings on
the Vega produced to the waviness standard of the german gliders then in
production. Presumably this was because the time consuming process of
sanding the wings after removal from the mould would have cost too much.

If this was the case then probably the Kestrel wings also were not sanded ?

Can anyone comment on the longevity of the finish on the Vega ?

Was it the material used on the Kestrel or the possible lack of sanding that
contributed to the lack of cracking problems ?

DB

A couple more data points for you *do-it-yourself* aficionados:

1. Re-establishing the original airfoil can be done by making some *quick &
dirty* templates, before starting. Wax and PVA (mold release) several locations
along the leading edge, say every 24 inches. Now make cardboard templates for
these locations, nothing fancy, just roughly the shape. With the L/E up, lay on
about 3 strands of glass rovings around the template locations. Next, pile on
some epoxy flox and shove your cardboard templates into the goo. Pop them off,
when cured and you have some exact replicas of your original leading edge
shape. I carry my templated back about 3 inches, as this is the most critical
area. Everything else is contouring to keep a smooth shape, both spsn-wise and
cord-wise.

2. You will need a good water trap in your feed line coming from the
compressor. I have used the *toilet paper* trap for years. It employs a roll of
toilet paper as the filter element. Just replace the roll before each major
operation (each wing) Some red-neck repairmen have even been known to dry out
the used rolls and then employ them again, for their original purpose. If you
don't have a good moisture trap, your spray gun will spit out little water
drops and they will show up as little craters on the product.
Have fun and remember, Everybody's got to be doing something. You have just
chosen to sand for the rest of your life.
JJ Sinclair

This thread has become very interesting…. there is considerable technical
discussion orbiting some of the comments. It would be nice if we were all in
the same room, where we could exchange a mass of information, rather than
simple RAS sound bites.

In too brief a sound bite fashion let me address a few of the comments:

The difference between boat gel coat and glider gel coat - It is a given that
boat gel coat holds up better than glider gel coat… many boat companies offer
a 10 year gel coat warranty! Boats are built with polyester gel coat and a
polyester (or vinyl ester variant) laminating resin substrate. The resulting
bond is a *COHESIVE* in nature. A previous comment was correct, that
essentially when boats come out of the mold they are finished (from a gel coat
cosmetic perspective - no sanding). The boats built today are using 4th and 5th
generation gel coat formulations that are fairly sophisticated. One small boat
company in the U.S. will use more gel coat than the entire glider industry.

Gliders, on the other hand, use polyester gel coat and an epoxy laminating
resin substrate. The resulting bond is an *ADHESIVE* force. The state-of-cure
of the gel coat applied in the mold at the time of laminate application is
critical to the bond. This is a complex interplay involving initiator level,
temperature, time, gel coat thickness, and other factors. There is an optimal
cure-state window for development of maximum bonding between the cured gel coat
and the laminate. At best, the adhesive bond between polyester and epoxy will
not produce as much energy as a polyester to polyester cohesive bond. The gel
coat typically used on gliders is the same basic 2nd or 3rd generation
technology as used on boats in the '70's and '80's. Also, there is
considerably more surface movement on a thin skinned glider laminate as
compared to a much thicker boat laminate.

Addressing a few of Ruben's comments - When gel coat is applied wet-on-wet, as
in the mold, it does cure as a uniform molecular matrix. When gel coat or a
paint coating is post-applied (as in repairs or refinishing leading edges out
of the mold) there is little, if any, crosslinking that occurs. This scenario
relies for the most part on a simple mechanical bond as Ruben correctly stated.

Ah, the moisture issue….. It is correctly stated that gel coat and
composites laminates have the properties of a semi-permeable membrane. However
the discussion leaves the tracks with the idea of liquid water penetration and
surface porosity. *Water vapor*, that is individual molecules of H2O, will
continually seek to equilibrate on the inner and outer skins of a laminate in a
very slow process. Water in the liquid state will *not* penetrate gel coat.
The surface pores and voids in the 3-D molecular matrix are too small for
liquid phase water to penetrate. This has to do with the inherent surface
tension of liquid H2O. The surface does not wet enough for liquid to flow into
the normal porosity.

You *do not* have to be concerned about washing your glider with water, or
leaving it out in the rain for that matter. It will not have a negative effect
on the gel coat. Wax does not seal in water. Vapor phase H2O will freely
equilibrate with no noticeable retardation of transmission through a wax film.
Additionally, since liquid water is not present within the gel coat or laminate
matrix, (under normal circumstances - let's not talk osmotic blisters), there
is no issue with freezing and causing cracks. This could become an issue with
giant cracks, but not with typical gel coat effects. Freezing water is simply
not an issue.

Again, hope this helps…. After 38 rounds this thread has stayed coherent….
has to be a record for RAS!

Bob Lacovara

Bob



If what you say about the 2nd or 3rd generation gel coat used in the boat of
the 70s and 80s is true and the assumption that moisture is not a problem
with gel coat how do you account for the horrible blister problems below the
waterline of many of the boats of the 70 and 80s. I experience this with
sailboat in the early 80s. The blister were full of liquid. A large number
of boats in the area where I sailed had this problem. It was not boat
manufacture specific.



Richard Pfiffner





"B Lacovara" wrote in message
...
This thread has become very interesting.. there is considerable technical
discussion orbiting some of the comments. It would be nice if we were all
in
the same room, where we could exchange a mass of information, rather than
simple RAS sound bites.

In too brief a sound bite fashion let me address a few of the comments:

The difference between boat gel coat and glider gel coat - It is a given
that
boat gel coat holds up better than glider gel coat. many boat companies
offer
a 10 year gel coat warranty! Boats are built with polyester gel coat and
a
polyester (or vinyl ester variant) laminating resin substrate. The
resulting
bond is a *COHESIVE* in nature. A previous comment was correct, that
essentially when boats come out of the mold they are finished (from a gel
coat
cosmetic perspective - no sanding). The boats built today are using 4th
and 5th
generation gel coat formulations that are fairly sophisticated. One small
boat
company in the U.S. will use more gel coat than the entire glider
industry.

Gliders, on the other hand, use polyester gel coat and an epoxy laminating
resin substrate. The resulting bond is an *ADHESIVE* force. The
state-of-cure
of the gel coat applied in the mold at the time of laminate application is
critical to the bond. This is a complex interplay involving initiator
level,
temperature, time, gel coat thickness, and other factors. There is an
optimal
cure-state window for development of maximum bonding between the cured gel
coat
and the laminate. At best, the adhesive bond between polyester and epoxy
will
not produce as much energy as a polyester to polyester cohesive bond. The
gel
coat typically used on gliders is the same basic 2nd or 3rd generation
technology as used on boats in the '70's and '80's. Also, there is
considerably more surface movement on a thin skinned glider laminate as
compared to a much thicker boat laminate.

Addressing a few of Ruben's comments - When gel coat is applied
wet-on-wet, as
in the mold, it does cure as a uniform molecular matrix. When gel coat or
a
paint coating is post-applied (as in repairs or refinishing leading edges
out
of the mold) there is little, if any, crosslinking that occurs. This
scenario
relies for the most part on a simple mechanical bond as Ruben correctly
stated.

Ah, the moisture issue... It is correctly stated that gel coat and
composites laminates have the properties of a semi-permeable membrane.
However
the discussion leaves the tracks with the idea of liquid water penetration
and
surface porosity. *Water vapor*, that is individual molecules of H2O,
will
continually seek to equilibrate on the inner and outer skins of a laminate
in a
very slow process. Water in the liquid state will *not* penetrate gel
coat.
The surface pores and voids in the 3-D molecular matrix are too small for
liquid phase water to penetrate. This has to do with the inherent surface
tension of liquid H2O. The surface does not wet enough for liquid to flow
into
the normal porosity.

You *do not* have to be concerned about washing your glider with water, or
leaving it out in the rain for that matter. It will not have a negative
effect
on the gel coat. Wax does not seal in water. Vapor phase H2O will freely
equilibrate with no noticeable retardation of transmission through a wax
film.
Additionally, since liquid water is not present within the gel coat or
laminate
matrix, (under normal circumstances - let's not talk osmotic blisters),
there
is no issue with freezing and causing cracks. This could become an issue
with
giant cracks, but not with typical gel coat effects. Freezing water is
simply
not an issue.

Again, hope this helps.. After 38 rounds this thread has stayed coherent..
has to be a record for RAS!

Bob Lacovara

This thread has become very interesting…. there is considerable technical
discussion orbiting some of the comments. It would be nice if we were all in
the same room, where we could exchange a mass of information, rather than
simple RAS sound bites.

In too brief a sound bite fashion let me address a few of the comments:

The difference between boat gel coat and glider gel coat - It is a given that
boat gel coat holds up better than glider gel coat… many boat companies offer
a 10 year gel coat warranty! Boats are built with polyester gel coat and a
polyester (or vinyl ester variant) laminating resin substrate. The resulting
bond is a *COHESIVE* in nature. A previous comment was correct, that
essentially when boats come out of the mold they are finished (from a gel coat
cosmetic perspective - no sanding). The boats built today are using 4th and 5th
generation gel coat formulations that are fairly sophisticated. One small boat
company in the U.S. will use more gel coat than the entire glider industry.

Gliders, on the other hand, use polyester gel coat and an epoxy laminating
resin substrate. The resulting bond is an *ADHESIVE* force. The state-of-cure
of the gel coat applied in the mold at the time of laminate application is
critical to the bond. This is a complex interplay involving initiator level,
temperature, time, gel coat thickness, and other factors. There is an optimal
cure-state window for development of maximum bonding between the cured gel coat
and the laminate. At best, the adhesive bond between polyester and epoxy will
not produce as much energy as a polyester to polyester cohesive bond. The gel
coat typically used on gliders is the same basic 2nd or 3rd generation
technology as used on boats in the '70's and '80's. Also, there is
considerably more surface movement on a thin skinned glider laminate as
compared to a much thicker boat laminate.

Addressing a few of Ruben's comments - When gel coat is applied wet-on-wet, as
in the mold, it does cure as a uniform molecular matrix. When gel coat or a
paint coating is post-applied (as in repairs or refinishing leading edges out
of the mold) there is little, if any, crosslinking that occurs. This scenario
relies for the most part on a simple mechanical bond as Ruben correctly stated.

Ah, the moisture issue….. It is correctly stated that gel coat and
composites laminates have the properties of a semi-permeable membrane. However
the discussion leaves the tracks with the idea of liquid water penetration and
surface porosity. *Water vapor*, that is individual molecules of H2O, will
continually seek to equilibrate on the inner and outer skins of a laminate in a
very slow process. Water in the liquid state will *not* penetrate gel coat.
The surface pores and voids in the 3-D molecular matrix are too small for
liquid phase water to penetrate. This has to do with the inherent surface
tension of liquid H2O. The surface does not wet enough for liquid to flow into
the normal porosity.

You *do not* have to be concerned about washing your glider with water, or
leaving it out in the rain for that matter. It will not have a negative effect
on the gel coat. Wax does not seal in water. Vapor phase H2O will freely
equilibrate with no noticeable retardation of transmission through a wax film.
Additionally, since liquid water is not present within the gel coat or laminate
matrix, (under normal circumstances - let's not talk osmotic blisters), there
is no issue with freezing and causing cracks. This could become an issue with
giant cracks, but not with typical gel coat effects. Freezing water is simply
not an issue.

Again, hope this helps…. After 38 rounds this thread has stayed coherent….
has to be a record for RAS!

Bob Lacovara

B Lacovara wrote:

The boats built today are using 4th and 5th
generation gel coat formulations that are fairly sophisticated. One small boat
company in the U.S. will use more gel coat than the entire glider industry.

Gliders, on the other hand, use polyester gel coat and an epoxy laminating
resin substrate. The resulting bond is an *ADHESIVE* force. The state-of-cure
of the gel coat applied in the mold at the time of laminate application is
critical to the bond. This is a complex interplay involving initiator level,
temperature, time, gel coat thickness, and other factors. There is an optimal
cure-state window for development of maximum bonding between the cured gel coat
and the laminate. At best, the adhesive bond between polyester and epoxy will
not produce as much energy as a polyester to polyester cohesive bond. The gel
coat typically used on gliders is the same basic 2nd or 3rd generation
technology as used on boats in the '70's and '80's. Also, there is
considerably more surface movement on a thin skinned glider laminate as
compared to a much thicker boat laminate.

Would gliders avoid the problem if they used the 4th and 5th generation
gel coat formulations? If so, why have the glider companies chosen to
use an older technology?

Ian,

My old LS-4 was partially refinished in Prestec about 5 years before I
bought it. It was clearly an Earl Scheib special (for those of you outside
the US - Earl Scheib = cheap auto refinisher of marginal quality). Within
2 years of my purchase, significant checking appeared in several areas.
Inspection with a 4x loop seemed to indicate that the crazing came from the
substrate (old gelcoat) not the outer surface. I think it just reinforces
the conventional wisdom that any areas of loose or badly crazed gelcoat need
to be removed prior to any sort of refinish.


"Ian Forbes" wrote in message
...

Does anybody out their own a ship which was refinished without removing
all
the gel-coat? What is the service history of these gliders?

I believe that Vegas are painted rather than gelled,
I have seen poor paintwork on Vegas though.

John