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Old 21-01-2016, 23:44   #1
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Estimating materials for bottom painting

From what I read here and there, bottom painting is a reasonable DIY job albeit not a fun one. I've estimated we need 6-700 square feet per coat, including daggerboard and rudders, and according to Pettit's application instructions we need some quantity of their "special" 120 Thinner for cleaning off sanding residue after sanding (but where we bought the paint, that particular thinner wasn't in stock.

My two inexperienced questions are:
What generic thinners should be compatible for the purpose (I have a hard time believing their is all that special)?
And more importantly, how the heck much thinner should I need, 2 quarts, 2 gallons, I have no idea?

Thanks
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Old 22-01-2016, 00:31   #2
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Re: Estimating materials for bottom painting

It be worth talking to Pettit, assuming that you can get a straight answer out of them. Meaning one not dictated by company policy, in order to make you buy only their stuff. Or worse, generated by lawyers.
That said, when you talk to them (& others who know paints), ask if the thinner in question also acts as something of a primer, or tie coat. Providing a chemical "tooth" to which the bottom paint can better tie in to.


BTW: Sweet boat you've got there.
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Old 22-01-2016, 02:26   #3
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Re: Estimating materials for bottom painting

If the thinners isn't available, wax and grease remover would be fine for cleaning hulls after sanding. I'd simply hose it off first, then when it's completely dry, wipe over with wax and grease remover.


A litre in a bucket for each hull would do it.
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Old 22-01-2016, 06:11   #4
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Re: Estimating materials for bottom painting

I'm sure some will consider this heresy, but my entire life I've never done more than hose the dust off and let it dry before bottom painting.
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Old 22-01-2016, 06:23   #5
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Re: Estimating materials for bottom painting

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Originally Posted by FSMike View Post
I'm sure some will consider this heresy, but my entire life I've never done more than hose the dust off and let it dry before bottom painting.
NOOOOOOOO!!!!

Well... It woks for ya... so on second thought.... Don't change a thing!
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Old 22-01-2016, 06:25   #6
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Re: Estimating materials for bottom painting

Acetone is my go to prep wipe...
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Old 22-01-2016, 10:15   #7
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Re: Estimating materials for bottom painting

First piont, since you are new to bottom paint, is that the stuff is quite tixic. Recommendation is not to dry sand. If you do then wear a good mask not just a little paper one, a disposable coverall is also good. Second, you don't say if it is hard or eroding A/F. Hard is more like standard paint but soft is much stickier! Third, what is already on and have you checked compatibility. If you change type of paint sometimes you need a barrier coat. The solvents in A/F are pretty aggressive and can soften the paint underneath so you finish up with paint that never dries, crazes or other nastiness. If you are not sure do a test on a small area. Final point is about mixing, a stick does not do it, you need a mechanical agitator or a mixer on a drill if it has stood for any time
Having said all that, yes it is a DIY job. I take any rough stuff off with a course flap wheel (despite the advice) but do where a mask and wash the dust off, as others say, (use a pressure washer when possible). On boats up to 35ft I have done it 'between tides' leading against a wall. Prep one tide, paint the next then turn her round and do the other side. Min two coats and three on the high wear areas. Not had any problems.
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Old 22-01-2016, 10:18   #8
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Re: Estimating materials for bottom painting

Yep - acetone is good stuff. As long as you remember that it's highly toxic and highly flammable.

Best approach, IMO, is power-wash to get all the large, slimey ***** off. Keep a tarp under/around the cradle to catch it so you can dispose of it as per local regulations. Then hand-wash with a strong Tri-Sodium Phosphate (TSP) solution to get the small stuff off. Use impermeable gloves, or your hands will be telling you about it! TSP is dirt cheap and VERY effective. Hose down with plain water and let dry. That gets you ready for painting.

With a rag moistened in the thinners that belongs to the paint, wipe down a few square feet just before you apply the paint to those few square feet. For quick results one person wipes, the other follows behind painting. Keep rotating/replacing the rag so the muck it picks up doesn't get spread around.

With modern paints it's best to stick to the recommended thinners cos you don't want chemical incompatibilities. In the old days, it didn't matter what you used. Mineral spirits or turpentine usually did the trick, but these days I think it does matter.

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Old 22-01-2016, 10:22   #9
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Re: Estimating materials for bottom painting

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Originally Posted by FSMike View Post
I'm sure some will consider this heresy, but my entire life I've never done more than hose the dust off and let it dry before bottom painting.
Sand and dust. Has worked just fine for 30 years. What is the solvent removing?I assume it was power washed first, so the salts are gone. The only possible function is to make the dust rag a little stickier. Where would wax or grease have come from? I think the manufacturer recommendation is boiler plate and CYA carried over from topsides recommendations, nothing more. Perhaps, like breaking matches to make certain they are cold, it is just a way to ensure the entire hull was actually dusted.

Re. Solvents, what does the paint smell like? what does the MSDS say is in it? I have used a dash of xylene (only a tiny amount should ever be needed--you don't thin bottom paint) with many brands over a 30 year period. Never a problem. It is not like you are reducing it for spraying.

This is bottom painting, not topsides and not epoxy work.
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Old 22-01-2016, 10:22   #10
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Re: Estimating materials for bottom painting

Quote:
Originally Posted by rgesner View Post
From what I read here and there, bottom painting is a reasonable DIY job albeit not a fun one. I've estimated we need 6-700 square feet per coat, including daggerboard and rudders, and according to Pettit's application instructions we need some quantity of their "special" 120 Thinner for cleaning off sanding residue after sanding (but where we bought the paint, that particular thinner wasn't in stock.

My two inexperienced questions are:
What generic thinners should be compatible for the purpose (I have a hard time believing their is all that special)?
And more importantly, how the heck much thinner should I need, 2 quarts, 2 gallons, I have no idea?

Thanks
Pretty much any solvent will work.

Water, will remove water soluble contaminants.

Petroleum based, some other contaminants.

Alcohol based, some other contaminants.

Most important is to ensure no residue that is incompatible with the paint, or inhibits the adhesion is used.

For this reason, paint manufacturers recommend everyone use their solvents and cleaners.

If you choose do something else, and have an issue, this will be their first defenses in denying a warranty claim.
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Old 22-01-2016, 10:25   #11
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Re: Estimating materials for bottom painting

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Originally Posted by TrentePieds View Post
Yep - acetone is good stuff. As long as you remember that it's highly toxic....

TrentePieds
No. Acetone is one of the least toxic solvents around. Flammable, yes, toxic, not hardly.

(from Wiki)
Health information

Acetone has been studied extensively and is generally recognized to have low acute and chronic toxicity if ingested and/or inhaled.[38] Acetone is not currently regarded as a carcinogen, a mutagenic chemical or a concern for chronic neurotoxicity effects.[37]
Acetone can be found as an ingredient in a variety of consumer products ranging from cosmetics to processed and unprocessed foods. Acetone has been rated as a generally recognized as safe (GRAS) substance when present in beverages, baked foods, desserts, and preserves at concentrations ranging from 5 to 8 mg/L.[38]
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Old 22-01-2016, 10:33   #12
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Re: Estimating materials for bottom painting

After sanding, I washed boat bottom with water and soap solution and boat brush, then rinsed well with water. Works in dry Mexico. Want to make sure that if you use water, surfaces are bone dry before painting however.
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Old 22-01-2016, 10:34   #13
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Re: Estimating materials for bottom painting

Quote:
Originally Posted by FSMike View Post
I'm sure some will consider this heresy, but my entire life I've never done more than hose the dust off and let it dry before bottom painting.
+1
Also I just use turps for thinning and cleaning up. Antifoul stays on fine.
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Old 22-01-2016, 11:21   #14
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Re: Estimating materials for bottom painting

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Originally Posted by HappyMdRSailor View Post
Acetone is my go to prep wipe...
Mine too Happy, Mine too.
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Old 22-01-2016, 11:25   #15
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Re: Estimating materials for bottom painting

I assume this is not a new boat. You don't need thinner. The last day of prep just wash the bottom good with fresh water, dry overnight and paint the next day. All you are really trying to get rid of is all the dust.
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