Urethane Paint

On Monday, November 16, 2015 at 4:59:07 PM UTC-5, Papa3 wrote:
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

welcome to my world. orange peel will be my death at last. i see it on everything.

On Thursday, November 19, 2015 at 3:13:45 PM UTC-5, ND wrote:
On Monday, November 16, 2015 at 4:59:07 PM UTC-5, Papa3 wrote:
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

welcome to my world. orange peel will be my death at last. i see it on everything.

Orange peel and grey shadows. As one goes, other comes.
Gloom
UH

Modern cars finish is all some degree of orange peel.

It uses a lot of very expensive paint to block and finish an entire
glider to a zero imperfection state. Did I mention the days and days of
patient marker coat eradication...

But it DOES look pretty when you are done.

On 2015-11-19 22:13, ND wrote:
On Monday, November 16, 2015 at 4:59:07 PM UTC-5, Papa3 wrote:
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

welcome to my world. orange peel will be my death at last. i see it on everything.


--
Bruce Greeff
T59D #1771

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.