Urethane Paint

On 11/15/2015 7:47 AM, 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?

When I apply Occam's razor to gel-coated, commposite gliders, I see relatively
flexible structure beneath relatively brittle gel coat, the latter which -
unsurprisingly, to me - tends to crack. Think chocolate-covered licorice
twists (or some such) - which material's gonna crack first?

The thicker the gelcoat, the more bending strain through its depth, and the
more likely a place for cracking to appear, but I wouldn't bet my life bending
and stiffness are the only contributors. In any event, polyurethane paint now
has a long history of being far more crack-resistant in glider use than every
gel-coat employed to date.

Some years ago, my club sanded one of our G-103's wing-pairs down to the
bottom of its very many (top & bottom/"classic pattern") cracks (because a
glass-inexperienced-annual-guy refused to sign off on it), then had an auto
shop shoot polyurethane. In some places we went down to the glass; in others
we did not. No cracks *into* the glass were found. In the 4 years or so before
we sold the - regularly used in wave - ship, nary a crack reappeared on any
painted surface.

Bob - chief sanding grunt - W.

On Sunday, November 15, 2015 at 10:23:42 AM UTC-5, wrote:

Where do the cross wise cracks and checking come from, based on what you have "heard"?

I've no idea. All I heard was that cracks start in the direction of the final sanding. The person that said it is very well regarded in the glider finishing world.

So does factory PU over factory gel coat delay cracking or have I just been lucky so far? I honestly don't know. I saw a badly trashed gel coat on an east coast only (with no O2 system) glider the other day and it got me wondering what the future holds for my nicely finished Polish factory PU'd wings. We both store our gliders under identical conditions.

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

UH...Curious as to the factory steps taken to apply a PU finish. Is everything the same (same gelcoat, same thickness, same sanding, same prep, et cetera sans waxing) then the wing is taken to a paint booth and sprayed like a car?

After spray painting, are the wings sanded or "rubbed" in any way?

What is the current "favorite" maintenance wax or polish for factory finished PU?

Thanks for your response.

On Sunday, November 15, 2015 at 5:24:23 PM UTC-5, wrote:
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

UH...Curious as to the factory steps taken to apply a PU finish. Is everything the same (same gelcoat, same thickness, same sanding, same prep, et cetera sans waxing) then the wing is taken to a paint booth and sprayed like a car?

After spray painting, are the wings sanded or "rubbed" in any way?

What is the current "favorite" maintenance wax or polish for factory finished PU?

Thanks for your response.

I have not seen a factory production painted glider produced. Andrzej could better comment on this. I do know that the quality of prep before paint needs to be much better than when finishing in gel coat because the paint is much thinner.
I'm sure there is color sanding and polishing.
We protect our painted gliders with Nu Finish glaze which seems to hold up pretty well.
UH

About 10 years ago when I asked a UK agent/ repairer for one of the major German companies about this I was told that they used thin gelcoat with less white pigment added before spraying with PU on their new gliders. The less pigment the better the polymerisation during curing I was told and colour density wasn't important as it was covered with PU. The gelcoat still looked completely white to me where I could see it under chips and scratches but that's what I was told.

On Thursday, November 12, 2015 at 2:15:39 AM UTC-5, Jonathan St. Cloud wrote:
On Wednesday, November 11, 2015 at 11:17:59 AM UTC-8, ND wrote:
On Tuesday, November 10, 2015 at 9:15:39 PM UTC-5, Jonathan St. Cloud wrote:
Any idea how much weight the urethane paint option adds to an 18 meter glider?

not enough different from gel-coat to matter unless you are really heavy or really light.

I was hoping to get a real number, i.e., 5, 10, 15 pounds, from someone with knowledge in this area. Not to be flippant nor disrespectful, but { not enough ... to matter, unless it does matter due to weight of the pilot} while thoughtful is really a null statement.

i apologize if you took my comment as flippant. i meant what i said. i have done 30+ refinishes and its been a mixture of gelcoat and PU refinishes. i do feel strongly that the weight difference is negligible.

Perhaps you can help me by answering why it is of interest to you?

are you close to CG or all up limits?

On Sunday, November 15, 2015 at 5:24:23 PM UTC-5, wrote:
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

UH...Curious as to the factory steps taken to apply a PU finish. Is everything the same (same gelcoat, same thickness, same sanding, same prep, et cetera sans waxing) then the wing is taken to a paint booth and sprayed like a car?

After spray painting, are the wings sanded or "rubbed" in any way?

What is the current "favorite" maintenance wax or polish for factory finished PU?

Thanks for your response.

I used to work in the paint shop at schempp-hirth.

the two processes are quite different.

What they do for paint is this: they apply a thin layer of molding gelcoat to the molds. it provides excellent adhesion to the composites, and releases from the mold nicely. when the fuse/wing/whatever is pulled from the mold, the thin layer of molding gel-coat is scuffed with sandpaper. at that point they then spray urethane primer, wetsand it, then do a PU topcoat, wetsand it. then its ready for trim stripes/ decals and polishing. after any striping, they also offer clearcoat to remove the lip edge of any decals/contest letters et cetera. but that's only applied in the local area of the decal.

With gelcoat the process is the same, except after the mold they just spray gelcoat onto the scuffed molding gelcoat.

On Sunday, November 15, 2015 at 5:24:23 PM UTC-5, wrote:
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

UH...Curious as to the factory steps taken to apply a PU finish. Is everything the same (same gelcoat, same thickness, same sanding, same prep, et cetera sans waxing) then the wing is taken to a paint booth and sprayed like a car?

After spray painting, are the wings sanded or "rubbed" in any way?

What is the current "favorite" maintenance wax or polish for factory finished PU?

Thanks for your response.

I used to work in the paint shop at schempp-hirth.

the two processes are different.

What they do for paint is this: they apply a thin layer of molding gelcoat to the molds. it provides excellent adhesion to the composites, and releases from the mold nicely. when the fuse/wing/whatever is pulled from the mold, the thin layer of molding gel-coat is scuffed with sandpaper. at that point they then spray urethane primer, wetsand it, then do a PU topcoat, wetsand it. then its ready for trim stripes/ decals and polishing. after any striping, they also offer clearcoat to remove the lip edge of any decals/contest letters et cetera. but that's only applied in the local area of the decal.

With gelcoat, after the mold they just spray gelcoat onto the scuffed molding gelcoat.

Thanks for your response.

I used to work in the paint shop at schempp-hirth.

the two processes are different.

What they do for paint is this: they apply a thin layer of molding gelcoat to the molds. it provides excellent adhesion to the composites, and releases from the mold nicely. when the fuse/wing/whatever is pulled from the mold, the thin layer of molding gel-coat is scuffed with sandpaper. at that point they then spray urethane primer, wetsand it, then do a PU topcoat, wetsand it. then its ready for trim stripes/ decals and polishing. after any striping, they also offer clearcoat to remove the lip edge of any decals/contest letters et cetera. but that's only applied in the local area of the decal.

With gelcoat, after the mold they just spray gelcoat onto the scuffed molding gelcoat.

Good to know...thanks ND.

How thick do you recon they get the PU topcoat? I know the PPG Concept we've use recommends two wet coats of 3 mils each for a total of 6 (this is for their standard auto applications). This then dries to a very thin (1-1.5 mils). We were able to get thicker coats for maybe 8-9 mil wet which seemed to give us (barely) enough material to color sand (wet sand). The automotive guy who did the Urethane paint couldn't understand why in the world we'd want to mess up his perfectly good spray job (with only very minor orange peel) by sanding!

As a sidebar... now that I've messed with this stuff, I can't help but look carefully at every car I see in the parking lot. I was looking at a Mercedes GLK the other day which had what glider guys would consider "bad" orange peel on a factory finish. On the other hand, from a work perspective, it's much more efficient to just "spray and forget it".

p3