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Old 15-12-2017, 22:06   #1
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Acid washing topsides of GRP boat

A broker told me of a boat that's been refitted to look like new. He went on to say that the topsides were acid washed then polished and they are "gleaming".
I was of the opinion that acid washing fibreglass boats damages the gelcoat.

Any comments would be appreciated.
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Old 15-12-2017, 23:46   #2
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Re: Acid washing topsides of GRP boat

Muratic acid
It cleans off all of the oxidation. Then you just rewax it
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Old 15-12-2017, 23:55   #3
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Re: Acid washing topsides of GRP boat

When I was working with fibreglass we made tanks to hold very strong acids. But as you may know there are many different kind of resins around. Some better than others.
here is a link at random: Storing Sulfuric Acid Using Fiberglass as a Material of Construction - Beetle Plastics

Damaging gelcoat? Doubt it, but firstly of course depends what acid is used. I think the common ones for cleaning are Oxalic acid, Phosphoric acid and Citric acid. For cleaning purposes on boats, I have only used the former one.

In my view, they do not affect the gelcoat at all. Yes these acids will remove any wax that is on the hull. The acid may leave some white residue, if the cleaned parts are not properly rinsed. These acids remove rust stains and the brown 'haze' close to the waterline.

As the diluted acid drips down the hull (ie from the deck, rigging, or railing) they drip then onto and along the antifoul. The antifoul can then show whitish streaks. In theory it may reduce the effects of the antifoul, but in practice, I have never noticed diminished efficiency of the antifoul.
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Old 16-12-2017, 05:32   #4
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Re: Acid washing topsides of GRP boat

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Originally Posted by HankOnthewater View Post



As the diluted acid drips down the hull (ie from the deck, rigging, or railing) they drip then onto and along the antifoul. The antifoul can then show whitish streaks. In theory it may reduce the effects of the antifoul, but in practice, I have never noticed diminished efficiency of the antifoul.
Diluted is the key word here. I wouldn't go at a boat with full strength acid. I do smaller sections at a time so the acid isn't on their too long.

I've acid washed my bandit and 420 several times. Makes them look great every year, like I keep them under a shelter and take care of them. They actually look like they don't spend summers on the shore behind the house and winter's under some pine trees, uncovered at all times.
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Old 16-12-2017, 05:43   #5
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Re: Acid washing topsides of GRP boat

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Muratic acid
It cleans off all of the oxidation. Then you just rewax it
YEP those toilet bowl cleaners
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Old 16-12-2017, 07:17   #6
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Re: Acid washing topsides of GRP boat

Mary Kate ON/OFF is nothing but acid. Super at cleaning gelcoat, including getting rust stains out. You dilute it pretty aggressively. Pretty much every yard I've been to uses it or it's equivalent to clean decks.
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Old 16-12-2017, 07:49   #7
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Re: Acid washing topsides of GRP boat

Older gelcoat gets porous, and picks up a brown mustache from things like tanin in the ICW. The fix is to acid wash with oxalic and then fill the pores with wax. Repeat as often as needed.
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Old 16-12-2017, 08:21   #8
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Re: Acid washing topsides of GRP boat

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YEP those toilet bowl cleaners
muratic is an ingredient in many crapper cleaners. I purchase mine full strength by the gallon.( cleans the dink of barnacles really good to.)
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Old 16-12-2017, 08:38   #9
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Re: Acid washing topsides of GRP boat

It should be clear from this that while the acid doesn't remove good gelcoat, it does remove the oxidized gelcoat. This in turn sets it up for further oxidation, so over time gelcoat is lost. Acid cleaning without sealing with wax or resin will probably accelerate gelcoat loss, so definitely keep up the seal as much as possible.

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Old 16-12-2017, 09:33   #10
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Re: Acid washing topsides of GRP boat

On/Off is just acid, diluted muriatic, I think. It is almost necessary for getting waterline tannin off white hulls before waxing. I do have something to add, however. In the islands, when you hire a local to wash and wax or just to wash, in some of the boatyards, what they do is splash muriatic or Ospho, then wax. No elbow grease for them. However, they pit your rubrails, your toerails, your through-hull fittings. We need to specify what can be used, or completely supervise these people.
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Old 16-12-2017, 09:36   #11
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Re: Acid washing topsides of GRP boat

oxalic acid is not very dangerous, I use it for years
let it bleach the hull few minutes, then rinse thoroughly, polish lightly, and apply a wax.
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Old 16-12-2017, 13:13   #12
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Re: Acid washing topsides of GRP boat

Muriatic acid can be purchased cheaply where swimming pool chemicals are sold. It's really just another name for hydrochloric acid, but when sold as muriatic acid it's usually more dilute.

Oxalic acid can be found as "Wood Bleach" at the hardware store. It's a box of white crystal powder that you mix with water. "Barkeeper's Friend" contains oxalic acid.
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Old 16-12-2017, 15:08   #13
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Re: Acid washing topsides of GRP boat

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Originally Posted by newhaul View Post
muratic is an ingredient in many crapper cleaners. I purchase mine full strength by the gallon.( cleans the dink of barnacles really good to.)
What final percentage solution do you use for cleaning gelcoat?

Around here bulk muratic is sold at 33%.
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Old 16-12-2017, 15:25   #14
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Re: Acid washing topsides of GRP boat

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What final percentage solution do you use for cleaning gelcoat?

Around here bulk muratic is sold at 33%.
I use it full strength just don't let it get on painted surfaces you don't want stripped clean off.
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Old 16-12-2017, 15:39   #15
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Re: Acid washing topsides of GRP boat

Hi There

The fibreglass tanks you used to store acids were they made using epoxy resin?

Happy days Sailing
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