PU paint not sticking

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PU paint not sticking

Postby Pasi in Finland on Fri Jan 26, 2024 6:02 am

I have now tried painting the boat bottom twice with the same depressing results:

I am using Hempel Polygloss, a reputable manufacturer 2-part Polyurethane paint. At the bottom there is a manufacturer recommended 2-part epoxy primer that is sanded smooth. The first layer goes on well on both of my attempts, but on both cases the second layer does not stick to the first.

Frustrating to say the least. Not to mention fairly costly and very laboursome.

Any ideas why this is happening? Especially on the second try I was careful to follow the instructions extra carefully. The inside temp was/is around 16 C (within the required minimum of 10 C by a large margin) I painted the second coat 24 hours after the first. Same paint but different manufacturing lot number.

I contacted the manufacturer, and they confirmed they have no reports of either batch being bad and did not have a definite answer as to what happened. Their only thought was that the 24 h was maybe not enough to evaporate all the thinner if the air circulation was not adequate. Hard for me to judge but my only hope right now.

Any advice anyone might have before I try the third time to sand everything, mark, paint etc? Even encouraging words might help to keep me from getting depressed :-)

Pasi

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Pasi in Finland
 
Posts: 27
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Re: PU paint not sticking

Postby dbeck on Fri Jan 26, 2024 3:13 pm

Pasi,

uhps. Unexpected and very frustrating indeed. I feel sorry.

We basically did the same thing. Two-part epoxy primer, then PU two-part paint.

What you describe sounds reasonable. The only thing I could image going terribly wrong is to confuse the types of hardener (in our case, there were DIFFERENT sorts of hardener for epoxy primer and PU paint). I believe the humidity meets specifications? Just in case you added thinner, was it the correct one for two-part PU paint?

You checked the mixing ratio? In our case the two part PU bilge paint (we used in the cabin) and the two part PU finishing paint (outside) require a different mixing ratio.

After each coat we waited about 2 days and sanded with 400 grid paper prior applying the next coat. We always add about 5% thinner to the paint. We did the painting between 15-20 degree Centigrade with garage door open. This worked out well and we will be doing the same again when painting the upper part of our boat. We bought primer and paint from here, https://www.bootsservice-behnke.de/cont ... /d139.html .They also advised us to go for two part paint. But the Pocketship build manual recommends one part paint.

Carry on!

Dietrich

EDIT: When we painted our lower hull, we were actually a bit lazy. We painted all the white in one go as one surface. The two coats of the blue stripes are actually painted on top of the two coats of white paint. Thus, at the blue stripes we actually have FOUR layers of paint and never had that problem. But in all cases we did wait at least for two days, then sanded, and then applied the next coat.
dbeck
 
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Re: PU paint not sticking

Postby Pasi in Finland on Sun Jan 28, 2024 1:53 pm

Thank you for the support Dietrich!

I am quite certain the hardener and thinner are as recommended. Especially on the second failed round I double and triple checked everything to make sure there are no mistakes. I really made a big effort to follow the instructions to the letter.

Just now I tried extending the time between the two coats with the rudder and painted the first coat Wednesday and the second Saturday to have a three day curing period. No improvement, the top layer peeled of easily today :-(

I cannot figure this out. Is the paint layer too thick? Too thin? Too much thinner? I confess I am most likely using a bit too much thinner as the paint is not smoothing out very well, but that should cause sagging and streaking, not this peeling I'm getting.

Very very frustrating and depressing.

Pasi
Pasi in Finland
 
Posts: 27
Joined: Mon Dec 07, 2020 7:37 am

Re: PU paint not sticking

Postby dbeck on Mon Jan 29, 2024 10:27 am

Pasi in Finland wrote:...
I cannot figure this out. Is the paint layer too thick? Too thin? Too much thinner? I confess I am most likely using a bit too much thinner as the paint is not smoothing out very well, but that should cause sagging and streaking, not this peeling I'm getting.
...


All this sound really strange. As you follow all instructions your problem might be related to some chemistry. Before proceeding on the boat, I would use a small piece of epoxied plywood and apply two coats of paint WITHOUT primer. If that works, take another piece of epoxied plywood and try with different products of primer and paint. Or give up on the two-part paint and use the one-part products from International that are suggested by CLC.

Sorry, I can't help.

Dietrich
dbeck
 
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Re: PU paint not sticking

Postby [email protected] on Mon Jan 29, 2024 12:55 pm

I have seen situations, due to bad prep or using incompatible primer and topcoat, where the top coat adhesion was poor,
and the paint tended chip off (or get pulled off by masking tape). But you case is worse: the adhesion is essentially ZERO,
and the topcoat is practically leaping off the surface. There is no way that some minor error like using the wrong amount of thinner
could cause this.
[email protected]
 
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Location: Princeton, NJ

Re: PU paint not sticking

Postby Pasi in Finland on Tue Jan 30, 2024 2:54 am

thanks for the support!

Very frustrating and depressing indeed.

I have read the paint datasheet an instructions over and over trying to see where I might have gone wrong and I can't figure this out. I've bought new paint from a different store and verified it is of a different lot number so it can't be the paint. It must be something I'm doing. I have spray painted a few cars and a number of motorcycles not to mention a few houses, so painting, mixing, importance of good preparation etc is clear to me. But this is my first try on 2-part PU and boats and it is not going well :-)

I would like to try a different manufacturer paint like International Perfection, but it seems the availability of 2-part PU paints is severely limited in EU, now the can says "professional use only". Hempel is the only such paint I can find in Finland.

My plan now is to sand everything and start over. Paint the first layer, let it dry then lightly sand and paint again. More work but definitely better adhesion.

Pasi
Pasi in Finland
 
Posts: 27
Joined: Mon Dec 07, 2020 7:37 am

Re: PU paint not sticking

Postby dbeck on Tue Jan 30, 2024 2:10 pm

Pasi,

about availability of paint: You might check for epifanes. This is a manufacturer for paint and related finishing products in Europe. They have a lot of guides and recommendations on their website too. Epifanes two-part paint is available from svb and according to their website svb ships to other countries within Europe.

In any case it might be worth asking for support at CLC and fyneboats (I believe you bought your kit there just as us). We had contacted them a couple of times and both are very helpful.

Dietrich
dbeck
 
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Re: PU paint not sticking

Postby Pasi in Finland on Wed Jan 31, 2024 1:31 am

Thanks gentlemen for your support!

I definitely need to check Epiphanes and if I could get some from overseas. I have read good things about it. There is a bit of a risk in shipping during the winter, as the delivery guy might leave the package on my doorstep while it is -20 outside and the paint freezes.

I am now thinking I'll do one more experiment on the rudder: I will paint the second layer only 7-8 hours after the first. The spec calls for min 8 hours @ 20 C (70 F) or 16 h @ 10 C (58 F). My workshop is 16C. Could it be that the paint dries faster then expected as the relative humidity is around 20 %?

What gets me is that I'm a fairly accomplished spray painter, and originally I was thinking if I should spray paint of roll&tip the boat. I decided on roll&tip because it was "faster and easier". Little did I know....

Pasi
Pasi in Finland
 
Posts: 27
Joined: Mon Dec 07, 2020 7:37 am

Re: PU paint not sticking

Postby Pasi in Finland on Mon May 20, 2024 7:27 am

To bring closure to this issue for the benefit for the future reader, I'll update what happened:

I ended up painting the boat three times total with Hempel Polygloss. First time -> didn't stick. "I must have done something wrong" and repainted following instructions to the letter -> didn't stick yet again. The third time having consulted with the paint manufacturer and being extra super careful with everything and carefully and diligently sanding the first layer I could not fail. The second coat definitely stayed better but not as it should shredding off fairly easily from the first coat. I must say it was more then depressing.

I sanded one more time and changed the paint manufacturer to Tikkurila, which is a very respected local Finnish brand and used their Temadur 90 2-part PU paint. It is meant for ships, bridges and the likes, but also recommended for boats. Strictly "professional use only" due to the isocynide containing hardener. Followed the instructions and this time things went easily with three coats and no sanding in between. Looks good and seems to be very durable.

So all is good. I am now convinced the Hempel Polygloss paint was bad in some way. It is good to know that the manufacture of this paint has ended and only old lots are available, with both the paints lots I tried within the last 6 months of its shelf life. I think it is a good guess that the shelf life Hempel has indicated is too optimistic and my paint lots had gone bad.

Pasi
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Pasi in Finland
 
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Re: PU paint not sticking

Postby Dmitriy on Mon Jun 24, 2024 6:40 am

Hello, Pasi!
How much Temadur paint did you spend for the hull?
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