PU finish in wave flying

[Eric]

Can anyone tell from experience how well a PU finish holds when flying in wave conditions, i.e. low temperatures & strong turbulence?
I'm considering to repaint my glider, remove old gelcoat > apply T35 as filler > polyurethane on top. Can I go wave-flying again with peace of mind? To what extent is a PU finish vulnerable to cracking due to change in temperature? There's tons of information on the refinish topic, but I couldn't find the answer to this particular question.

Thanks!

[[email protected]]

On Wednesday, January 29, 2020 at 11:16:57 AM UTC-5, Eric wrote:
> Can anyone tell from experience how well a PU finish holds when flying in wave conditions, i.e. low temperatures & strong turbulence?
> I'm considering to repaint my glider, remove old gelcoat > apply T35 as filler > polyurethane on top. Can I go wave-flying again with peace of mind? To what extent is a PU finish vulnerable to cracking due to change in temperature? There's tons of information on the refinish topic, but I couldn't find the answer to this particular question.
>
> Thanks!

T35 is polyester "gelcoat". That is the stuff that cracks. I would remove all "gelcoat", fill and surface with thin layer of polyester spray filler, then paint in UU/PU.
Good luck
UH

[Dan Marotta]

My Stemme, finished in PU and 16 years old still has a perfect finish
except for chips and scratches.Â* I fly in wave quite often.

On 1/29/2020 9:24 AM, wrote:
> On Wednesday, January 29, 2020 at 11:16:57 AM UTC-5, Eric wrote:
>> Can anyone tell from experience how well a PU finish holds when flying in wave conditions, i.e. low temperatures & strong turbulence?
>> I'm considering to repaint my glider, remove old gelcoat > apply T35 as filler > polyurethane on top. Can I go wave-flying again with peace of mind? To what extent is a PU finish vulnerable to cracking due to change in temperature? There's tons of information on the refinish topic, but I couldn't find the answer to this particular question.
>>
>> Thanks!
> T35 is polyester "gelcoat". That is the stuff that cracks. I would remove all "gelcoat", fill and surface with thin layer of polyester spray filler, then paint in UU/PU.
> Good luck
> UH

--
Dan, 5J

[Eric Greenwell[_4_]]

Dan Marotta wrote on 1/29/2020 9:28 AM:
> On 1/29/2020 9:24 AM, wrote:
>> On Wednesday, January 29, 2020 at 11:16:57 AM UTC-5, Eric wrote:
>>> Can anyone tell from experience how well a PU finish holds when flying in wave
>>> conditions, i.e. low temperatures & strong turbulence?
>>> I'm considering to repaint my glider, remove old gelcoat > apply T35 as filler
>>> > polyurethane on top. Can I go wave-flying again with peace of mind? To what
>>> extent is a PU finish vulnerable to cracking due to change in temperature?
>>> There's tons of information on the refinish topic, but I couldn't find the
>>> answer to this particular question.
>>>
>>> Thanks!
>> T35 is polyester "gelcoat". That is the stuff that cracks. I would remove all
>> "gelcoat", fill and surface with thin layer of polyester spray filler, then
>> paint in UU/PU.
>> Good luck
>> UH
>
> My Stemme, finished in PU and 16 years old still has a perfect finish except for

> chips and scratches. I fly in wave quite often.

It's not the wave flying that does it, of course, but the low temperatures. What
OAT do you typically see while wave flying, and what is the lowest OAT you've seen
while flying the Stemme?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[Eric]

PU is also used in the automotive industry. Metal shrinks/expands more under temperature changes than our glasfiber wings do. Is it too easy to say that if polyurethane works on cars than it certainly works on gliders?

I don't intent to do the paintjob myself, but I'll go to a professional EASA approved workshop, and they use T35 as groundlayer. Would that affect the crack resistance (against temp changes) in a negative way?

[[email protected]]

On Wednesday, January 29, 2020 at 1:50:37 PM UTC-5, Eric wrote:
> PU is also used in the automotive industry. Metal shrinks/expands more under temperature changes than our glasfiber wings do. Is it too easy to say that if polyurethane works on cars than it certainly works on gliders?
>
> I don't intent to do the paintjob myself, but I'll go to a professional EASA approved workshop, and they use T35 as groundlayer. Would that affect the crack resistance (against temp changes) in a negative way?

AU or PU is more flexible than "gelcoat" and thus less prone to crack.
UH

[Dan Marotta]

Single digits, say 5 deg F.

On 1/29/2020 10:46 AM, Eric Greenwell wrote:
> Dan Marotta wrote on 1/29/2020 9:28 AM:
>> On 1/29/2020 9:24 AM, wrote:
>>> On Wednesday, January 29, 2020 at 11:16:57 AM UTC-5, Eric wrote:
>>>> Can anyone tell from experience how well a PU finish holds when
>>>> flying in wave conditions, i.e. low temperatures & strong turbulence?
>>>> I'm considering to repaint my glider, remove old gelcoat > apply
>>>> T35 as filler > polyurethane on top. Can I go wave-flying again
>>>> with peace of mind? To what extent is a PU finish vulnerable to
>>>> cracking due to change in temperature? There's tons of information
>>>> on the refinish topic, but I couldn't find the answer to this
>>>> particular question.
>>>>
>>>> Thanks!
>>> T35 is polyester "gelcoat". That is the stuff that cracks. I would
>>> remove all "gelcoat", fill and surface with thin layer of polyester
>>> spray filler, then paint in UU/PU.
>>> Good luck
>>> UH
>>
> > My Stemme, finished in PU and 16 years old still has a perfect
> finish except for
>
> > chips and scratches.Â* I fly in wave quite often.
>
> It's not the wave flying that does it, of course, but the low
> temperatures. What OAT do you typically see while wave flying, and
> what is the lowest OAT you've seen while flying the Stemme?
>

--
Dan, 5J

[Eric Greenwell[_4_]]

Eric wrote on 1/29/2020 8:16 AM:
> Can anyone tell from experience how well a PU finish holds when flying in wave conditions, i.e. low temperatures & strong turbulence?
> I'm considering to repaint my glider, remove old gelcoat > apply T35 as filler > polyurethane on top. Can I go wave-flying again with peace of mind? To what extent is a PU finish vulnerable to cracking due to change in temperature? There's tons of information on the refinish topic, but I couldn't find the answer to this particular question.

The Kiwis have used PU longer than most. Decades ago, they would order their
gliders with the thinnest gel coat the manufacturer (Schleicher, etc) would
provide, and without any sanding or or polishing. Then they'd sand and paint them
in PU to get a finish that would survive their frequent high and lengthy wave flights.

So, contact a Kiwi, get information, and report back!

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1

[[email protected]]

Check with the Perlon Project folks. I'll bet their paint is PU. I believe it has experienced temperatures in excess of -80 F.

[[email protected]]

As I understand the problem, it is not so much the gelcoat as the difference in thermal expansion/contraction rates between the surface coat (Gel or PU) and the underlying composite structure (Fiberglass/Carbon). As the temperature changes rapidly (fast ascent or descent through a significant temperature gradient), the two materials expand or contract at different rates. Too rapid a temperature change can cause cracks to form in the thin and fragile topcoat.

Bob LaCovara (sp?) gave a very good lecture at (I believe) the 2006 SSA Convention in Ontario, CA that addressed this and other interesting facts about composite structures and finishes. One fact he pointed out was that there are many different types of "gelcoat," and the sailplane industry has been a very tiny segment of the market, utilizing gelcoat formulations that are nowhere near the technical superiority of more modern products, and what the industry does use is often applied incorrectly and in a much thinner layer than what was intended by the manufacturer.

Indeed, the different types of gelcoats used by various sailplane manufacturers often exhibit widely disparate results in longevity and appearance. Some of this even manifests itself between examples of the same make and model glider depending on how the gelcoat was originally applied in the mold. In the 80's, it was assumed that there wasn't any difference between laying the first glass fiber and epoxy resin into place immediately, or a day after the gelcoat was sprayed in the mold. Only after fifteen or so years would it become apparent that crazing and cracking was more common in the gliders that had a delay of more than a few hours between spraying the coating and sealing it from the air with the fiberglass layers.

[Eric Greenwell[_4_]]

wrote on 1/29/2020 5:51 PM:
> Check with the Perlon Project folks. I'll bet their paint is PU. I believe it has experienced temperatures in excess of -80 F.
>
I'm not sure what the paint is (might be on their website), but the composite is
almost entirely "pre-preg" carbon, which might have different properties than the
wet layup used in most of our gliders.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorgliders/publications/download-the-guide-1