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Old 10-10-2011, 16:44   #1
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Mud Cracks in Bottom Paint

Hi all... the previous owners sanded back to the barrier coat and put three coats of bottom paint (at one time) at least 2 years ago and it's been on the hard for a year.

To be fair it looks in good shape in most places, but I have noticed what a friend calls 'mud cracks' along the top edge (and through about 6 or eight inches down) where the sunlight can hit it through the water... and it's quite thick right there which I'm told also leads to the problem.

It literally just chips away right back to the glass. I can certainly sand these areas back and repaint them if that's the best thing to do... but I guess I'm a little concerned that the new coat will also crack off at the barrier coat elsewhere on the hull.

How do I know were the damaged areas extend to? And how do I know if I need a completely new coat everywhere - and if I DO need a whole new coat, do I have to sand down the entire hull back to glass since this current paint is so thick?

Sorry - don't have a photo but can take one when I'm back at the yard tomorrow.

Cheers!
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Old 10-10-2011, 18:08   #2
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Re: mud cracks in bottom paint

Welcome to the joys of boat ownership!

Take a scraper to the paint--if it flakes off easily, it all has to come off. Otherwise it will take your new coat of paint with it. If most of it is sticking, then sand the edges to feather them out.

If you do take it back to the barrier coat, roll a coat of epoxy on, then put the bottom paint on when the epoxy has hardened to the 'green' stage (den'ts with a fingernail). That will ensure that the bottom paint won't flake off again.
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Old 10-10-2011, 18:52   #3
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Re: mud cracks in bottom paint

"Mud cracks" and "it chips away right back to the glass" lead me to believe that what you are seeing may be a case of failed chemical bonding of the gelcoat to the laminate in the mold. This is fairly common, the manufacterers just wait a little too long and lose the chemical bond, often in a particular area where the gelcoat cured too much before being laminated on. I've seen cases where the whole boat was like this and cases where it was just a small area. If it really chips back to the glass and not the gelcoat this is the most likely scenario. If it chips back to gelcoat, it's probably because they tried to use a non-sanding primer when they commissioned it and this is causing the barrier coat to fail; much less likely. The "mud cracks" could also be a sign of gelcoat alligation, either in factory gel laps or in a repair. When you chip it off, do you go back to clean glass with no gelcoat at all on it? Some pics would help. If you want to do an adhesion test for paint, the standard method is to really rub on some duct tape in a big patch and then cut an X in it through the paint to the substrate. Then pull the tape aggresively. If paint flakes off around the edges of the cut the adhesion test failed. If you end up with a nice clean X cut in your bottom paint it's good.
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Old 10-10-2011, 19:28   #4
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Re: mud cracks in bottom paint

OK.. sorry, it chips off to the gelcoat not the glass (was typing it out too fast). There is a blue layer that I was led to believe is a barier coat of 2 part epoxy type paint then a thick layer of black antifoul. When it chips, it chips at the white gelcoat (which is smooth and in perfect condition) eg: it's the blue layer that comes off.

Now I may be thinking that is not a barrier coat, but just old blue anti-foul which has not adhered as well or has lost it's adhesion over many years. The black bottom paint that the PO applied is sticking fine to the blue layer... it's the blue layer that's cracking and then subsequently chipping as I sand.
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Old 10-10-2011, 20:18   #5
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Re: mud cracks in bottom paint

I've kept my boat on the hard for 95% the time for the past 10 years. I too get the mud cracks, on the sunny side mostly. I just scape off whats lose and slap on some fresh B-paint over the gaps and smaller cracks each spring before a launch.

It keeps until I'm back on the hard again. Basically, it's dried paint shrinking and loosing it's bond. But when I hits the water permanently I'll do a full strip and repaint.
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Old 10-10-2011, 20:55   #6
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Re: mud cracks in bottom paint

Yup, that's just old dried up bottom paint. I'd do a bottom paint removal, take the whole thing back to gelcoat. At that point you might as well slap on 4-5 coats of 2000 as well before you anti-foul. No big deal...
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Old 10-10-2011, 21:00   #7
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Re: mud cracks in bottom paint

Quote:
Originally Posted by Aussie_Sequoia View Post
OK.. sorry, it chips off to the gelcoat not the glass (was typing it out too fast). There is a blue layer that I was led to believe is a barier coat of 2 part epoxy type paint then a thick layer of black antifoul. When it chips, it chips at the white gelcoat (which is smooth and in perfect condition) eg: it's the blue layer that comes off.

Now I may be thinking that is not a barrier coat, but just old blue anti-foul which has not adhered as well or has lost it's adhesion over many years. The black bottom paint that the PO applied is sticking fine to the blue layer... it's the blue layer that's cracking and then subsequently chipping as I sand.
Duratec vinylester barrier coat is blue, but it's a light powder blue. If what you are seeing is dark blue it probably is bottom paint, I don't know of a barrier coat that's dark blue. And I've never seen one fail by cracking. Is it fairly soft when you sand? Barrier coat is generally pretty hard.
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