Paint Blisters

[Plantman]

I have just recovered my plywood boat from a swinging mooring(fresh water) where it has been for the last 3 months. Above the waterline the paint is in near perfect condition however below the waterline It is covered in small blisters which, when pierced, contain water. The hull was painted all over with marine gloss, above & below the waterline. The bubbles do not appear to go down to the ply but only to the original surface that I first painted over. Although I had antifouled the boat, in a rush, just before launching I am sure this is nithing to do with it. A lot of the antifouling has come off & the gloss purely visble is blistered. Also some antifouing that had not been constantly submerged is fine.
What's gone wrong?

[warthog5]

Not uncommon. If you knew that the boat would sit in the water. It should have not been painted with any other paint than antifoul.
The fix is to strip the bottom and repaint with antifoul.

[Kev Smyth]

Yup- all paints except anti-foul will blister if left immersed, even the fancy two or three part urethanes. If you leave the blisters alone, they most often will dry out and disappear until the boat is again left wet. It takes 2-3 weeks for the bubbles to appear, depending on the water temp. And longer for them to dry out. Trailer/beach the boat, use anti-foul, or just live with it. ;)

[paul oman]

only a handful of paints and coatings are designed for constant immersion, certainly not enamels, urethanes, latex, etc.

paul oman
progressive epoxy polymers

[sdowney717]

Well I can tell you from experience I never had anti foul blister when against bare wood or even overcoating old antifoul.
I had been using mostly abaltive type paint.
Now I have switched to a new idea of using Sanitred Permaflex against the bare wood. This stuff is a urethane and is positively 100% waterproof and actually soaks into clean bare wood. It wont blister and it will keep the hull dry and sealed. It is like a glossy rubber sheet on the wood. It does not stick to oily wood very well.
It feels very slick when wetted with water. Since antifoul cost is high, the permaflex cost at about 75$ a gallon is comparable and I get to seal my hull and I wont have to repaint with antifoul They claim and others have since showed me marine growth wont stick very well to it. I was skeptical at first but even the US /navy at their Trident base showed this to be true. I know a lot of you will totally doubt this to be true.

[Plantman]

Well you learn by your mistakes!! The reason I applied gloss was because last season it was used as a trailer sailer & after taping the seams with epoxy & tape I just brought it back to what it was before, Gloss!
I gather now that the only course of action is to strip the hull back to bare ply then what? Do I prime it again then antifoul it direct or add an undercoat? Also while the hull is back to bare wood is there any advantage in applying epoxy, either with matting or just the resin in an attempt to protect the wood furthur?

[sdowney717]

http://www.sanitred.com/permatechdata.htm

Going back to wood would be a perfect time to use a urethane sealer. I dont know if you could get this easily in the UK. This stuff is really working out well for me.