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Old 30-09-2017, 05:56   #1
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Do I need to strip my bottom?

I'll upload pictures hopefully later today, but in the meantime, let me try to describe the situation.

I've been sanding my bottom paint with 60-grit for the last few days and am about complete.

There are some areas that were a bit flaky, and I sanded these until the paint stopped lifting and then faired the area. There are other areas that show an appearance of "alligator skin". I've also been hitting these spots hard, and though they feel smooth to the touch, you can still see the alligator skin. Then the are other areas that are perfectly fine.

My plan was to finish with this sanding, paint with a bottom primer, and then put the new anti-fouling on. I think the new antifouling is perfectly compatible with the old one, but there were a few spots I did some glass repairs and gel-coated.

What are you guys' thoughts on having to strip the old paint?
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Old 30-09-2017, 07:42   #2
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Re: Do I need to strip my bottom?

I'd advise removing all the old paint, inspect the gelcoat for osmosis, if not present, barrier coat and bottom paint.
If you do have an osmosis issue you will have a little work ahead of you.
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Old 30-09-2017, 08:09   #3
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Re: Do I need to strip my bottom?

Some of the answer depends on how much paint you'd still need to remove if you were to sand it all off. It's also worth doing adhesion testing of the paint which is still on there. The test with cross hatched squares & tape, as well as the solvent test, before & after the former.

That said, I'd be likely to fully strip the bottom, & get it stupid smooth, prior to applying new coatings.

The other key questions are; what type of bottom paint is on there now, what type will you be applying, & what does the new paint's manufacturer recommend?
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Old 30-09-2017, 08:23   #4
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Re: Do I need to strip my bottom?

Alligatored paint is where chips/chunks of paint have flaked off. If it has happened in some areas, it's going to happen in future in other areas as well. It's just a law of nature. "Fairing" the areas that currently show alligatoring may smooth out the surface but it does nothing to solve the underlying problem.

That said, it really is a matter of how much time and money you want to spend addressing it and how important the result is to you. There is nothing "wrong" with alligatored paint...it's not going to sink the boat. If you haul and paint every two years you could follow your current routine indefinitely and all you'd lose in the bargain is a knot or so of boat speed.

But if you take it all down now and redo the bottom in earnest future maintenance will be much, much easier. You'll have a smooth fair bottom and you can keep it that way by not allowing paint build up. Sanding and repainting will be relatively quick and easy each time you haul, if you do it right.

If you take it all off you can then inspect the gelcoat. If you can't find barrier coat, then would be a good time to put it on, not only as it will protect the gelcoat from osmosis but it also acts as a primer; hot coat the first coat of anti-fouling paint over it and you get excellent adhesion and a good base for future paint. Where you have done repairs I would certainly barrier coat, even if you don't take the whole hull down.

Sanding old alligatored paint with 60 grit can be a fool's errand, depending on the type of paint and it's condition. Sometimes it just basically burnishes the paint. I would suggest going down to 36-40 grit. Before that I would actually go at it with a carbide scraper, which can often be easier than sanding. If you're confident and have a good "hand" for it, you can also take it off with a polisher/grinder with a 36 grit wheel on a soft pad.
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Old 30-09-2017, 09:29   #5
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Re: Do I need to strip my bottom?

I had similar issues with my boat and started the process you are on. I ended up taking off all the old paint, priming, and then painting. Allows you to totally own the end result and start "new." It was a beast, but one you'll likely never have to do again.
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Old 30-09-2017, 10:14   #6
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Re: Do I need to strip my bottom?

I didn't have the time & money to do it properly last year. That quick and dirty paint job I did then was just a waist of time & money as the new anti fouling went away rather quickly sometimes with parts of the old one... So I would suggest you do it clean if you can.

I shall do it properly next month myself. Question: is there a way we can scrap down to the old barrier coat or do we have to remove it all down to gelcoat ?
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Old 30-09-2017, 10:34   #7
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Re: Do I need to strip my bottom?

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I didn't have the time & money to do it properly last year. That quick and dirty paint job I did then was just a waist of time & money as the new anti fouling went away rather quickly sometimes with parts of the old one... So I would suggest you do it clean if you can.

I shall do it properly next month myself. Question: is there a way we can scrap down to the old barrier coat or do we have to remove it all down to gelcoat ?
Barrier coat, being an epoxy paint, is generally considerably harder than most bottom paints and harder to scrape off. It's also usually white, grey or green and so easy to see when you get down to it.

I scraped my boat down to the factory barrier coat, filled and faired nicks and flaws, and put a new barrier coat on for good measure, 8 coats and then sanding it smooth to prevent orange peel build up in subsequent layers. Then on went anti-fouling within the specified window so that it chemically bonded. I used a Sherwin-Williams commercial marine barrier coat (less than 50% the cost of Interlux or Pettit) for the first 7 coats, then Pettit's barrier coat for the last as I was putting on Trinidad. It's a good idea to use the same brand barrier coat and anti-fouling for the best compatibility.
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Old 30-09-2017, 12:27   #8
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Re: Do I need to strip my bottom?

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Originally Posted by Suijin View Post
Barrier coat, being an epoxy paint, is generally considerably harder than most bottom paints and harder to scrape off. It's also usually white, grey or green and so easy to see when you get down to it.

I scraped my boat down to the factory barrier coat, filled and faired nicks and flaws, and put a new barrier coat on for good measure, 8 coats and then sanding it smooth to prevent orange peel build up in subsequent layers. Then on went anti-fouling within the specified window so that it chemically bonded. I used a Sherwin-Williams commercial marine barrier coat (less than 50% the cost of Interlux or Pettit) for the first 7 coats, then Pettit's barrier coat for the last as I was putting on Trinidad. It's a good idea to use the same brand barrier coat and anti-fouling for the best compatibility.
Which Sherwin-Williams commercial marine barrier coat did you use? They have quite a list of paints.
I am about ready to strip my boat bottom and paint it. A 32' fiberglass boat with swing keel. How much paint do you think I would need to do the first 7 coats and then for the final barrier coat.
I am preparing myself for a tough job stripping it because I have only seen it out of the water once briefly when I bought it and it looked rough, as if they pressure washed it and painted it without fairing it.
We will be cruising the Gulf of Mexico and then the Bahamas.
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Old 30-09-2017, 13:46   #9
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Re: Do I need to strip my bottom?

I have a 26' powerboat in freshwater, so take my comment or leave it. I scraped, sanded, and faired the existing bottom paint for days which had not been touched in 5 years. All flaking and loose paint was removed and plenty of solvent was used before painting. Plenty of alligator skin remained, which in my situation, looks more like paint shrinkage than flaking. Just pulled the boat for the second season and I still have 100% adhesion. With proper prep, IMO, the paint will stick.

the quote for ecoblast was 900$. i had at least 30 hours of prep. Pretty sure it was worth my time to do it myself instead of paying for the ecoblast.

either way shizz still looks good.

cheers
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Old 30-09-2017, 13:47   #10
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Re: Do I need to strip my bottom?

I am away from my computer so cannot type in detail. However I have done a massive bottom job using Sherwin-Williams barrier and antifouling paint. More later.
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Old 30-09-2017, 14:01   #11
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Re: Do I need to strip my bottom?

Quote:
Originally Posted by keithasully View Post
Which Sherwin-Williams commercial marine barrier coat did you use? They have quite a list of paints.
I am about ready to strip my boat bottom and paint it. A 32' fiberglass boat with swing keel. How much paint do you think I would need to do the first 7 coats and then for the final barrier coat.
I am preparing myself for a tough job stripping it because I have only seen it out of the water once briefly when I bought it and it looked rough, as if they pressure washed it and painted it without fairing it.
We will be cruising the Gulf of Mexico and then the Bahamas.
MIL-24441 Type III, Epoxy Polyamide

Part A and part B come in 5 gallon buckets. I bought it through a yard and they pro rated the charge for just what I used, which probably ended up being 5-6 gallons for my 40'er.

You don't have to put 8 coats on, but it's "best practice", the idea being that you don't ever want to do it again and epoxy is really not completely waterproof, so the more you put on the better. The above product is a bit thinner than the retail products...you could get by with four coats of the Pettit stuff I suppose. It's tough, staring at the shelf and how much the stuff costs and contemplating buying a gallon for each coat...
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Old 30-09-2017, 14:12   #12
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Re: Do I need to strip my bottom?

I'm not a pro, but to me 8 coats of barrier seems a bit over the top (!) to me. Most of the pro folks seem to apply 2 or 3 coats. I doubt that the next 5 coats bring much advantage to the system.

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Old 30-09-2017, 14:34   #13
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Re: Do I need to strip my bottom?

For those contemplating stripping their bottoms, if you have several years of paint on there it adds up to a lot of thickness & weight. Think about how much just 1 gallon of bottom paint weights, & then multiply by X.

Also, in lieu of sanding, it's often possible to contract out to have the old paint media blasted off. Which if you have 3mm of paint on a 40'er, that's a rediculous amount of sanding. Even if scrapers & chemical strippers pull off a good bit of it.
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Old 30-09-2017, 15:42   #14
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Re: Do I need to strip my bottom?

Wherever I still had good paint from 2011 on the bottom, I left it. As far as the topside hull, I took it all the way down to the original 40 year old gelcoat.

The attached pictures show the sample before I sanded off all the old peeling paint from the topside hull.

The bottom got one coat, and the topside hull two.

This at a cost of maybe $425.00 (in the fall/winter of 2014/15) plus sand paper etc. I plan to do the bottom again next Fall/Winter and we'll see on the topside
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Old 30-09-2017, 15:55   #15
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Re: Do I need to strip my bottom?

Definitely cheaper than Interlux:

MIL-DTL-24441 Formula 150, Epoxy-Polyamide Paint, Green Primer, Type III and Type IV

TDS says one coat primer, 2 coats paint (over aluminum)

http://www.sticadhesive.com/stic-tds...ype-III-V2.pdf
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