Soda Blasting Hull: What Next??

[hriehl1]

1968 Hinterhoeller HR28

I am having the bottom soda blasted this week to remove 10 +/- annual coats of ablative bottom paint applied by the prior owner. I just want my ownership to start with a known clean condition. I expect it'll result in a smooth hull, no blisters or other surprises.

My question is, assuming no surprises, what do folks recommend next? Casey's book says don't apply epoxy barrier coats if there is no history of problems, and anecdotes of people removing barrier coats they applied 15 years ago that are now compromised also worries me. So I take an "if it ain't broke don't fix it" approach.

So if everything looks OK after the soda blast, I'm inclined just to apply ablative bottom paint, nothing more.

Comments? (I apologize for questions whose answers may seem obvious to you more-experienced owners, but they are not obvious to me.)

[Carl-A259]

Actually this is a great question that has been bothering me as well, Barrier coats seem to create their own problems. My thinking is a couple coats of Epoxy primer, then bottom paint, but I am also looking for the answer.

[LazyGuy]

If the bottom is good, the bottom is good. Assuming you know what color bottom paint you will be using, I would start with one or two coats of a compatible hard bottom paint of a contrasting color. Us it as an indicator coat. When you start thinking that you have too much paint building up, just paint a couple of coats where the indicator coat is showing. Eventually, the end of season power wash will show hints of the indicator coat over most of the bottom. A little bit of sand paper with a vacuum collection system will do the rest.

[Rachel]

Blisters/barrier coat is a topic with many opinions. I happen to think the hydrolysis idea probably has some merit, but I don't think it's something that "melts" a hull in a short period of time. Here is the basic idea of that theory, as I (a non-chemist) interpret it. I may be wrong:

All gelcoat is somewhat porous. On newer boats it is less so. What happens on boats that have blisters is that water moves in through the gelcoat and combines with styrenes left over from the fiberglass layup to make a new fluid (the smelly brown juice). This new fluid has larger molecules and so cannot pass back out through the gelcoat as readily, hence blisters form.

On older boats with more porous gelcoat, the new fluid can pass back out, so it does, and blisters do not form. Instead, that styrene is carried away. This is called hydrolysis. Theoretically, if the hydrolysis carried on indefinitely, the layup would become dry and starved, thus weaker -- although I don't think that's very easy to become an issue in a "real world" situation. That is, the hulls are probably becoming weaker but not at a hugely fast, "melting" clip. So does it matter in the practical life of the boat? I don't know on that one.

Barrier coats attempt to make the outer surface of the boat less porous, to eliminate the fluid transfer of the water in in the first place. If that happens, no blisters and no hydrolysis, presuming that it does work and is less porous.

I find it ironic that traditional wooden boats fare best in the water, but really, for fiberglass boats, you are doing the layup a favor by taking them out of the water and storing them in air.

Again, not everyone will agree on this, I'm no chemist, and I may change my mind if I learn something new; but it makes the most sense to me at this time.

Rachel

[mitiempo]

If the paint is ablative why did it build up enough for soda blasting to be necessary? It should slowly dissolve.

[Rachel]

I've seen ablative build up on boats that don't get out much. After some years they were soda blasted to get back to (in their case) the barrier coat and start over. (Not sure why they kept adding coats before that - maybe the ablative can lose efficacy before it "ablates" if the boat is not moving much? At any rate it had built up, and this was a half dozen boats not just one.)

[hriehl1]

The prior owner told me he had 10 years ablative paint on the hull, and prior to that, perhaps 10 years "vinyl paint"; whatever that is. I have no way to probe more explicitly what may be on there. so my preference is simply to soda blast to start my ownership with a known condition.

[mitiempo]

That makes sense.

[Rachel]

The prior owner told me he had 10 years ablative paint on the hull, and prior to that, perhaps 10 years "vinyl paint"; whatever that is.

That's interesting. Usually vinyl paint (such as VC-17) is one of those paints that you can't put anything over (the solvents in it lift other types of paint). Of course who knows what it is for sure after all this time. Probably not a bad idea to start fresh, with what you know.