Urethane Paint

5 kg. sounds like a decent ballpark estimate. My club has an '85 DG-300 which was repainted with polyurethane last year. The original gel coat had been cut polished twice and by last year was in pretty rough shape. The shop didn't remove all the old gel coat but just lightly sanded it to take care of the oxidation and crazing. Going by the before and after weighings it gained 3.1 kg.

At 05:11 14 November 2015, wrote:
5 kg. sounds like a decent ballpark estimate. My club has an '85
DG-300
whi=
ch was repainted with polyurethane last year. The original gel
coat had
bee=
n cut polished twice and by last year was in pretty rough shape.
The shop
d=
idn't remove all the old gel coat but just lightly sanded it to take
care
o=
f the oxidation and crazing. Going by the before and after
weighings it
gai=
ned 3.1 kg.


LS4 with very poor original gel taken to a professional refinisher (in
Poland). Aircraft gel completely stripped down to structure.
Reprofiled, T35 gel used as a filler/primer and finished in acrylic
paint (offered as option to PU). Aircraft returned looking beautiful
and ~ 1lb lighter.

Jim

On Tuesday, November 10, 2015 at 7:15:39 PM UTC-7, Jonathan St. Cloud wrote:
Any idea how much weight the urethane paint option adds to an 18 meter glider?

Not removing all the gelcoat will likely result, over time, in the cracks coming back through the urethane paint.

Removing all the gelcoat and then priming and spraying urethane paint should not result in a weight gain. Especially on older glider where there are some areas of very thick gelcoat.

It is all in how the work is done.

Robert Mudd
Composite Aircraft Repair
Moriarty, New Mexico.

On Saturday, November 14, 2015 at 10:43:25 AM UTC-8, Robert M wrote:
On Tuesday, November 10, 2015 at 7:15:39 PM UTC-7, Jonathan St. Cloud wrote:
Any idea how much weight the urethane paint option adds to an 18 meter glider?

Not removing all the gelcoat will likely result, over time, in the cracks coming back through the urethane paint.

Removing all the gelcoat and then priming and spraying urethane paint should not result in a weight gain. Especially on older glider where there are some areas of very thick gelcoat.

It is all in how the work is done.

Robert Mudd
Composite Aircraft Repair
Moriarty, New Mexico.

I believe the OP was asking about getting a new glider with the optional polyurethane in which case the factory sprays the paint over the gelcoat and not about a refinish.

On Wednesday, 11 November 2015 02:15:39 UTC, Jonathan St. Cloud wrote:
Any idea how much weight the urethane paint option adds to an 18 meter glider?

PU or Acrylic topcoat paint is just used as a very thin UV block, it is usually a very thin layer and is not used to "profile" the surface. If you are not concerned about the durability of your glider finish and want to maximise performance then just going with gelcoat would be the obvious choice... it also allows you to more easily reprofile the wings once they have done their "shrinking". Schempp seem to be the only glider manufacturer to largely avoid their wings looking like prunes after a few years.

If I was having a new glider I would have the fuselage, tailplane, control surfaces all done in PU and then get the wings and tip extensions painted a couple of years later once the shrinkage has stabilised and surfaces have been "profiled". If you do leave the wings bare then consider having the numbers done in cellulose so they can be easily removed prior to painting.

I would also have the underwing numbers clearcoated and faired in so there are no sharp edges left to mess with the airflow.

On Sunday, November 15, 2015 at 4:34:23 AM UTC-5, Paul Crabb wrote:

PU or Acrylic topcoat paint is just used as a very thin UV block, it is usually a very thin layer and is not used to "profile" the surface.

Does the PU protect the gelcoat from temperature related crazing?
Someone told me that gelcoat crazing was attributed to rapid change in temperature associated with flying wave. Once the cracks start moisture penetrates and accelerates the deterioration.

Anyone have a crazed PU painted wing?

Anyone have a better explanation for wing crazing?

In recent decades the temperature related cracking is an issue iparticularly affecting hard, otherwise durable, Schwabellack gelcoat on DG gliders. For other manufacturers using T35 and similar it is mostly UV and moisture that causes the degredation of the surface.

On Sunday, November 15, 2015 at 8:57:36 AM UTC-5, wrote:
For other manufacturers using T35 and similar it is mostly UV and moisture that causes the degradation of the surface.

I heard that the cracks develop in the direction of the final sanding (front to back). My understanding is that the sanding leaves microscopic cracks in the direction of the sanding, then moisture gets into the cracks and expands them. And the PU coating seals those microcracks and slows down the penetration of moisture and the subsequent gradual widening of the cracks?

On Sunday, November 15, 2015 at 9:47:30 AM UTC-5, son_of_flubber wrote:
On Sunday, November 15, 2015 at 8:57:36 AM UTC-5, wrote:
For other manufacturers using T35 and similar it is mostly UV and moisture that causes the degradation of the surface.

I heard that the cracks develop in the direction of the final sanding (front to back). My understanding is that the sanding leaves microscopic cracks in the direction of the sanding, then moisture gets into the cracks and expands them. And the PU coating seals those microcracks and slows down the penetration of moisture and the subsequent gradual widening of the cracks?

Where do the cross wise cracks and checking come from, based on what you have "heard"?
PU does seal that surface up better, mostly because it seals up the porosity in the gelcoat as well as being a much better UV barrier.
UH

The paint on my 14 year old LAK-17a looks factory. No cracking or
crazing whatsoever and only a few chips. I'm a believer!

On 11/15/2015 6:17 AM, son_of_flubber wrote:
On Sunday, November 15, 2015 at 4:34:23 AM UTC-5, Paul Crabb wrote:

PU or Acrylic topcoat paint is just used as a very thin UV block, it is usually a very thin layer and is not used to "profile" the surface.
Does the PU protect the gelcoat from temperature related crazing?
Someone told me that gelcoat crazing was attributed to rapid change in temperature associated with flying wave. Once the cracks start moisture penetrates and accelerates the deterioration.

Anyone have a crazed PU painted wing?

Anyone have a better explanation for wing crazing?

--
Dan, 5J