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27 Posts
Discussion Starter · #1 ·
A buddy of mine is preparing his boat (an older Grady) boat for a new application of bottom paint..He is going to go with a high quality ablative
paint. About 75% of his hull is down to
the gelcoat with maybe a deep nick here
and there. Is a coat of barrier coat on
the bare stuff a good idea???

· Premium Member
10,097 Posts
Barrier Coats are of some value on boats that have had a history of developing blisters on the hull bottom. These coatings are not the perfect solution to this problem because of two issues -

First - to be truly effective, the hull must be close to perfectly dry prior to application and this can only properly be checked with a fairly sophisticated instrument - called a moisture meter. Its not something that the average boater owns or has even ever seen, for that matter. Its a tool that a marine surveyor commonly employs to check hull soundness.

If the hull moisture content is too high, the barrier coating will not be effective - the blisters will reappear.

Second - the other issue is that even the guys that have done the barrier coat job the "Proper" by-the-book way frequently have blisters reappear. Why? Because the hull bottom isn't the only place that water can get into the hull. It can also enter via the bilge. In other words From the inside of the boat.

Many go thru this expensive, messy and time-consuming process only to find that the hull is just as badly blistered on the next haul-out because of a basic lack of knoweldge. How discouraging do you think that sight would be?

Not really their fault either - Interlux and Pettitt spend mucho bucks advertising these products to death and if you walk into any boat chandlery and ask about blisters to the average $9/hour high-school kids that seem to gravitate to employment in those places you will get the automatic stock answer - "Barrier coat the hull." Well, thats part of the answer, but not the entire solution.

If your friend's Grady has no blisters or only a very few blisters now and its a fairly "Seasoned" hull - say more than 5 seasons old, its unlikely that it will ever develop this problem. So here's what I would do - its what I did on my own rig after laboriously spending a very cold and miserable winter laying flat on my back under it stripping all 26 feet of bottom paint with an automotive windshield-sticker razor blade:

First - run a vibrating palm sander with 100-grit production paper mounted on it over the entire hull. This will give the hull some "Tooth" for the primer to grab.

Second - wipe the entire underside down with Interlux wax remover - work from bow to stern in long sweeps and bring along a good supply of rags - you want to push any of the old original mold-release wax residue to the rear and ultimately off the hull. Do not wipe in circular or back-and-forth motions - this will just smear it around.

Third - tell him to buy this stuff (Click here):

Interlux Interprotect Epoxy Filler

Or alternately even a pint of Marinetex and fill those deep nicks and/or gouges with either of those.

Let it chemically set (Overnight would be best) and then fair the filled areas flat with that same vibrating sander with 80 or 100 grit.

Fourth - now give the entire bottom a coat of this stuff:

Interlux No-Sand Primer

Fifth - follow the directions carefully, but generally when it gets really tacky you roll on your Bottom Paint right over it and it all tightens up to a real nice professional-looking job. (Usually at least two coats of bottom paint the first time around)

Here's the paint that I use on my boat:

Interlux Bottomcote ACT

I have found this stuff to be as effective as the much more expensive Micron ablative products at roughly half the price. I watch the Spring-time chandlery circulars like a hawk and when I see a good price I go get the stuff. Last month I paid $84/gallon at Fred Chall in Freeport - which is a pretty good price, based on what I've seen.

I paint and drop my boat in around April 1 every season and it doesn't come out till Jan 15, at least. Never ever do I find a barnacle on the hull, and I use the stuff right over all my inboard running gear as well.

I've used it now for 7 seasons - pretty much since its introduction to the market I believe - and the total paint build-up on the hull is less than 3/32 of an inch. Not quite paper thin, but WAY less than the 1/4" or more that an old-technology solid non-ablative would have left on there.

Oh and another nice thing about ablative paints - if the coating is getting a little thick after a period of years - a good washing with a high-powered pressure washer will knock the stuff right pack to the primer.

No more scraping, ever.

What an improvement.

So anyway, that's what I would advise your friend.

If he were my friend, that is.

rgds, Leprechaun

(This post edited by Leprechaun on 04/13/2003)


· Registered
1,621 Posts
flynny7, doesn't sound to me as though your friend's boat has a blister problem, I think your just asking how to re-paint the bottom of an old boat that has had the old bottom paint worn down to the hull in spots? If this is correct are you just asking if the barrier coat would be a good idea before you put bottom paint on it? If the boat is not having any blistering problems I don't think the barrier coat is necessary. Just sand it down as mentioned above. I'm not sure if you should prime the whole bottom over what old bottom paint is still hanging on, perhaps someone else could say if this is appropriate. I redid my 16'(much smaller area)and took off all the bottom paint right down to a clean hull (quite a job). Boat is a 1962 and if I was going to keep it I decided it should be done. Started with a scraper, then an orbital sander, finally with steel wool and paint remover. I used marine filler to patch the gouges then a barrier coat before priming and painting. When it was all done I think I impressed even myself but its not something I'd want to do again any time soon. If the hull is in decent shape and the old bottom paint is worn off 75% of the hull already why not remove the last 25% with marine paint remover, patch it, prime it and paint it. Your buddy will be pleased and feel secure his hull is protected. Once your into it the job becomes a labor of love. If he is looking to get it in the water as soon as possible and start fishing perhaps a sanding and two coats of bottom paint without any barrier coat would get him by, but the boat is a 'Grady' I would think he would want to take the time and do it right, he'll enjoy years of happy boating if he does.
just my $.02.

· Registered
13,099 Posts
flynny7, you didn’t mention anything about blistering, only that the hull has a nick here, or there. The barrier coat systems are designed for hulls with gel coat problems, i.e., blistering. Stay away from it, you don’t need it. As a side note, I would like to say that the barrier systems do work, however, they are extremely labor intensive, and you must follow the instructions 100%. The is a lot of preparation work involved and if the boat is kept outdoors, you need a very good long weather window which is often difficult to find this time of year.

If you have already removed 75% of the bottom paint, you may as well go for it and remove the rest. Fill any of the gouges with MarineTex, and fair with sandpaper accordingly. You can purchase small cans of gel coat if you want to cover the MarineTex if you like, but it is not necessary.

After the paint is removed, wash the underside hull down with soap and water, rinse well. Interlux has some solvents that you can then wipe down the hull with to remove any soapy, wax residue.

It is now time to paint the bottom. You may have noticed that I didn’t mention anything about sanding the bottom, to give it some “grip” as others have mentioned. I would not recommend doing this. Fiberglass and gel coat are porous and will absorb water. The gel coat is very thin, why wear it down? If the old paint didn’t come off that that would tell me the surface is “rough” enough to accept paint. If you are concerned about it, use the Interlux No-Sand Primer. The purpose of this product is so that you DON’T have to sand. I know you want to do the right thing, but don’t get anal about it, it’s the bottom of the boat. As far as paint, I would recommend Interlux Micron 2000, it’s just about the best on the market right now. Use 2 coats, each a different color. Next year you only touch up where the 1st. color is visible.

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