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.

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

On Sunday, November 15, 2015 at 6:17:38 AM UTC-7, 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?

I've just sanded away the old gelcoat off the wings of my 1978 LS3. I've noticed a couple interesting patterns regarding the gel coat crazing.

a) All crazing stopped abruptly at the edge of any covered portion of the wings gel coat. So for example, the gel coat that was covered by turbulator tape and Mylar tape was perfectly free of any crazing. The crazing lines dead ended at the boundary.

What this means to me, is that at least some portion of crazing is due to evaporation/drying/degradation over time of an exposed critical gel coat chemical. I imagine that some organic molecule embedded in the gel-coat composition over time breaks up and likes to go float around in the atmosphere. I know it is not the sun or UV causing breakup because the bottom surfaces of the wing and stab presented the same patterns and the same amount of crazing.

b) All crazing lines followed an underlying surface defect or discontinuity.. For example in my wings, stab, and fuse they followed previous sanding patterns perfectly. This makes sense as the deepest sanding gouges would naturally have the greatest density of exposed surface area and thus degrade the fastest. Or so my theory goes. Interestingly enough there was a perfect craze line that matched the exact pattern of the underside original owners initials that where painted on the wing, JB.

c) The crazing extended to the contact layer of the fiberglass/epoxy layer leaving visible troughs. This very thin contact layer was a distinct material between the gel coat and the fiberglass and was the yellow color of vanilla ice cream. I have no clue what this material is, although Dave Nelson suggested it could be the remaining layer of a previous gel coat. I also thought it could be the "Warning Colour" material called out in the LS3 Maintenance manual as:
Nitro Cellulose Kombilack Reinorange RAL 2004 made by Lackfabrik Bader on page 13.9.

Despite looking extensively through a jewelers magnifying glass I saw no evidence of the crazing cracks extending into the fiberglass matrix.

The only caveat is that Rolladen Schneider used a sandwich structure for the wings and stab skins on the LS3. Not sure if this is true for other sailplane manufacturers. I noticed the outer stab skin is very very thin and can be easily sanded into if you're not careful. According to the maintenance manual, the outer layer of the sandwich has only two layers.

I'd appreciate if you could comment on my findings/theory. One thing for sure, is that I could not identify a single crazing line on my LS3 that was due to mechanical stress or skin shearing like you typically find on the corners of the spoiler box on a Grob. I have some good pictures I'll post.

Roberto

On Monday, November 16, 2015 at 6:53:30 PM UTC-8, wrote:
On Sunday, November 15, 2015 at 6:17:38 AM UTC-7, 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?

I've just sanded away the old gelcoat off the wings of my 1978 LS3. I've noticed a couple interesting patterns regarding the gel coat crazing.

a) All crazing stopped abruptly at the edge of any covered portion of the wings gel coat. So for example, the gel coat that was covered by turbulator tape and Mylar tape was perfectly free of any crazing. The crazing lines dead ended at the boundary.

What this means to me, is that at least some portion of crazing is due to evaporation/drying/degradation over time of an exposed critical gel coat chemical. I imagine that some organic molecule embedded in the gel-coat composition over time breaks up and likes to go float around in the atmosphere.. I know it is not the sun or UV causing breakup because the bottom surfaces of the wing and stab presented the same patterns and the same amount of crazing.

b) All crazing lines followed an underlying surface defect or discontinuity. For example in my wings, stab, and fuse they followed previous sanding patterns perfectly. This makes sense as the deepest sanding gouges would naturally have the greatest density of exposed surface area and thus degrade the fastest. Or so my theory goes. Interestingly enough there was a perfect craze line that matched the exact pattern of the underside original owners initials that where painted on the wing, JB.

c) The crazing extended to the contact layer of the fiberglass/epoxy layer leaving visible troughs. This very thin contact layer was a distinct material between the gel coat and the fiberglass and was the yellow color of vanilla ice cream. I have no clue what this material is, although Dave Nelson suggested it could be the remaining layer of a previous gel coat. I also thought it could be the "Warning Colour" material called out in the LS3 Maintenance manual as:
Nitro Cellulose Kombilack Reinorange RAL 2004 made by Lackfabrik Bader on page 13.9.

Despite looking extensively through a jewelers magnifying glass I saw no evidence of the crazing cracks extending into the fiberglass matrix.

The only caveat is that Rolladen Schneider used a sandwich structure for the wings and stab skins on the LS3. Not sure if this is true for other sailplane manufacturers. I noticed the outer stab skin is very very thin and can be easily sanded into if you're not careful. According to the maintenance manual, the outer layer of the sandwich has only two layers.

I'd appreciate if you could comment on my findings/theory. One thing for sure, is that I could not identify a single crazing line on my LS3 that was due to mechanical stress or skin shearing like you typically find on the corners of the spoiler box on a Grob. I have some good pictures I'll post.

Roberto

The "warning color" is red - orange lacquer that's used as anti-collision markings.

There is typically filler along the leading edges where the two halves of the wing were joined. Also, your particular LS3 had some profiling work done in the area of the spar; is that where this additional material was found?

Currently have a 1984 LS4 on the operating table...

p3

On Monday, November 16, 2015 at 9:53:30 PM UTC-5, wrote:
On Sunday, November 15, 2015 at 6:17:38 AM UTC-7, 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?

Comment- Yep- where painting was done over surfaces where all compromised gelcoat was not removed and surface simply filled.

Anyone have a better explanation for wing crazing?

I've just sanded away the old gelcoat off the wings of my 1978 LS3. I've noticed a couple interesting patterns regarding the gel coat crazing.

a) All crazing stopped abruptly at the edge of any covered portion of the wings gel coat. So for example, the gel coat that was covered by turbulator tape and Mylar tape was perfectly free of any crazing. The crazing lines dead ended at the boundary.

Comment- Over coating, such as number painting, seals the surface and pretty much stops degradation.

What this means to me, is that at least some portion of crazing is due to evaporation/drying/degradation over time of an exposed critical gel coat chemical. I imagine that some organic molecule embedded in the gel-coat composition over time breaks up and likes to go float around in the atmosphere.. I know it is not the sun or UV causing breakup because the bottom surfaces of the wing and stab presented the same patterns and the same amount of crazing.

b) All crazing lines followed an underlying surface defect or discontinuity. For example in my wings, stab, and fuse they followed previous sanding patterns perfectly. This makes sense as the deepest sanding gouges would naturally have the greatest density of exposed surface area and thus degrade the fastest. Or so my theory goes. Interestingly enough there was a perfect craze line that matched the exact pattern of the underside original owners initials that where painted on the wing, JB.

Comment- This is why finish sanding needs to be very well done so as to not leave deep scratches which are initiation points for moisture, UV, and mechanical stress.

c) The crazing extended to the contact layer of the fiberglass/epoxy layer leaving visible troughs. This very thin contact layer was a distinct material between the gel coat and the fiberglass and was the yellow color of vanilla ice cream. I have no clue what this material is, although Dave Nelson suggested it could be the remaining layer of a previous gel coat. I also thought it could be the "Warning Colour" material called out in the LS3 Maintenance manual as:
Nitro Cellulose Kombilack Reinorange RAL 2004 made by Lackfabrik Bader on page 13.9.

Comment- This is likely to be a change in the surface characteristics of the gelcoat where the epoxy for laminating was applied wet on wet over the tacky gelcoat. Tis is done to get the best bond between two very different materials.

Despite looking extensively through a jewelers magnifying glass I saw no evidence of the crazing cracks extending into the fiberglass matrix.

The only caveat is that Rolladen Schneider used a sandwich structure for the wings and stab skins on the LS3. Not sure if this is true for other sailplane manufacturers. I noticed the outer stab skin is very very thin and can be easily sanded into if you're not careful. According to the maintenance manual, the outer layer of the sandwich has only two layers.

Comment- Almost all primary surfaces can be expected to be sandwich, except fuselages which are mostly thick single layer.

I'd appreciate if you could comment on my findings/theory. One thing for sure, is that I could not identify a single crazing line on my LS3 that was due to mechanical stress or skin shearing like you typically find on the corners of the spoiler box on a Grob. I have some good pictures I'll post.

Roberto

Comments inserted above- messy
UH

On Tuesday, November 17, 2015 at 8:30:21 AM UTC-6, wrote:
On Monday, November 16, 2015 at 9:53:30 PM UTC-5, wrote:
On Sunday, November 15, 2015 at 6:17:38 AM UTC-7, 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?

Comment- Yep- where painting was done over surfaces where all compromised gelcoat was not removed and surface simply filled.

Anyone have a better explanation for wing crazing?

I've just sanded away the old gelcoat off the wings of my 1978 LS3. I've noticed a couple interesting patterns regarding the gel coat crazing.

a) All crazing stopped abruptly at the edge of any covered portion of the wings gel coat. So for example, the gel coat that was covered by turbulator tape and Mylar tape was perfectly free of any crazing. The crazing lines dead ended at the boundary.

Comment- Over coating, such as number painting, seals the surface and pretty much stops degradation.

What this means to me, is that at least some portion of crazing is due to evaporation/drying/degradation over time of an exposed critical gel coat chemical. I imagine that some organic molecule embedded in the gel-coat composition over time breaks up and likes to go float around in the atmosphere. I know it is not the sun or UV causing breakup because the bottom surfaces of the wing and stab presented the same patterns and the same amount of crazing.

b) All crazing lines followed an underlying surface defect or discontinuity. For example in my wings, stab, and fuse they followed previous sanding patterns perfectly. This makes sense as the deepest sanding gouges would naturally have the greatest density of exposed surface area and thus degrade the fastest. Or so my theory goes. Interestingly enough there was a perfect craze line that matched the exact pattern of the underside original owners initials that where painted on the wing, JB.

Comment- This is why finish sanding needs to be very well done so as to not leave deep scratches which are initiation points for moisture, UV, and mechanical stress.

c) The crazing extended to the contact layer of the fiberglass/epoxy layer leaving visible troughs. This very thin contact layer was a distinct material between the gel coat and the fiberglass and was the yellow color of vanilla ice cream. I have no clue what this material is, although Dave Nelson suggested it could be the remaining layer of a previous gel coat. I also thought it could be the "Warning Colour" material called out in the LS3 Maintenance manual as:
Nitro Cellulose Kombilack Reinorange RAL 2004 made by Lackfabrik Bader on page 13.9.

Comment- This is likely to be a change in the surface characteristics of the gelcoat where the epoxy for laminating was applied wet on wet over the tacky gelcoat. Tis is done to get the best bond between two very different materials.

Despite looking extensively through a jewelers magnifying glass I saw no evidence of the crazing cracks extending into the fiberglass matrix.

The only caveat is that Rolladen Schneider used a sandwich structure for the wings and stab skins on the LS3. Not sure if this is true for other sailplane manufacturers. I noticed the outer stab skin is very very thin and can be easily sanded into if you're not careful. According to the maintenance manual, the outer layer of the sandwich has only two layers.

Comment- Almost all primary surfaces can be expected to be sandwich, except fuselages which are mostly thick single layer.

I'd appreciate if you could comment on my findings/theory. One thing for sure, is that I could not identify a single crazing line on my LS3 that was due to mechanical stress or skin shearing like you typically find on the corners of the spoiler box on a Grob. I have some good pictures I'll post.

On Wednesday, November 11, 2015 at 3:15:39 AM UTC+1, Jonathan St. Cloud wrote:
Any idea how much weight the urethane paint option adds to an 18 meter glider?

I repainted my Ls1 wings last winter. I removed about 10kg of gelcote, then did I added about 6kg of primer and PU-paint.

Regarding the odd color on the LS3, I had the same on my LS1. On the first LS-gliders was the gelcote and fiberglas laid wet-on-wet, which made the gelcote and epoxy to be mixed. That mixture has an cream/yellow color and penetrates into the first layer of firbeglass. Do not grind though this as it would genereate a lot of extra work by adding a new layer of firbreglas.

On Tuesday, November 17, 2015 at 7:50:39 AM UTC-7, Per Carlin wrote:
On Wednesday, November 11, 2015 at 3:15:39 AM UTC+1, Jonathan St. Cloud wrote:
Any idea how much weight the urethane paint option adds to an 18 meter glider?

I repainted my Ls1 wings last winter. I removed about 10kg of gelcote, then did I added about 6kg of primer and PU-paint.

Regarding the odd color on the LS3, I had the same on my LS1. On the first LS-gliders was the gelcote and fiberglas laid wet-on-wet, which made the gelcote and epoxy to be mixed. That mixture has an cream/yellow color and penetrates into the first layer of firbeglass. Do not grind though this as it would genereate a lot of extra work by adding a new layer of firbreglas.

Thanks, between your answer and that of UH, I think my question regarding the odd contact layer has been put to bed. I didn't realize that the fiberglass was layered into the mold on top of wet Gel.

I still haven't made a decision on brand of primer and PU paint to use. I'm curious to know what you used. Regarding the PU paint, there seems to be a consensus on PPG Concept, but it is absurdly expensive around here ($450 a gallon. Regarding the primer, I've seen many mentions of "PCL polyprimer" and on the car business side of things "Evercoat Slick Sand".