View Full Version : Hull Cleaners-What do those chemicals do to the fiberglass?

06-28-2005, 04:06 PM
I've been searching for something to get the brown Chesapeake Bay stain off of my hull. I looked at Mary Kate on&off as well as FSR and toilet bowl cleaner as recommended by a friend. All of them contain heavy concentrations of phosphoric acid and/or oxalic acid. The on&off recommends that you cover the trailer in plastic or it will dissolve the galvanizing! If it's that nasty, what's it doing to my gelcoat? Does anybody know if these cleaners cause long term damage to the gelcoat?

06-28-2005, 04:50 PM

The gelcoat is pretty much impervious to inorganic acids. That's not to say that you can drown the hull in the stuff with impunity. Oxalic and Phosphoric acids are relatively weak (as opposed to Muriatic or Sulfuric acids).

Where I used to work, we used marine epoxy resins and fiberglass to make tanks to contain strong cleaning acids (far more concentrated than consumer strength hull cleaners) and they lasted for several years bathed 24/7 in some nasty stuff at temps of around 140 F.

What you have to remember is that Galvanize is a Zinc coating and Zinc is a fairly active metal (low nobility). It is easily attacked by acids. Zincs are used to preserve the other, more important, metal items in boats (Brass, cast Iron, etc.). Throw a pencil Zinc in some Muriatic acid and see what happens to it.

You're going to spray or rub on some hull cleaner, let it get rid of the Tannin stains (prolly in 30 seconds or so) and then wash it off. Also, if you wax your hull those weak acids won't have a prayer of damaging the gelcoat since they will hardly even penetrate to it. I wouldn't use toilet bowl cleaner because there are some organics in there that may attack the gelcoat (IIRC they tell you to use that stuff on Porcelin surfaces only, not on fiberglass or acrylic tubs or sinks).

I remember taking a trip from Turkey Point down to Chestertown and could see the patches of reddish-brown water (some really dark) as we travelled down the Eastern shore of the bay. When we got back and hauled the boat, I was apalled at the grungy brown stains on the hull and boat wash wouldn't even touch them. I got some hull cleaner from BoatUS and had it cleaned up in about 15 minutes. I did splash some on one of the trlr fenders and was amazed at how fast the cleaner attacked the Galvanize. So, if you use the stuff, you do need to protect the trlr.

Capt Crunch
06-28-2005, 05:57 PM
Garry's wax with a random-orbit buffer takes it off pretty good.

mel slidell
06-28-2005, 08:07 PM
Heck! Check the label on a can of coke, they use phosporic acid in every can of it! By the same token, I've been selling it to Readymix Companies for removing cement off the trucks to get them clean. So!!!! Don't worry Go have a Coke while you clean your boat!
Mel slidell

06-28-2005, 10:00 PM
I've used muriatic acid from Lowe's and Home Depot on my hull and you can't tell the difference.

I will say... I had a '66 Chevelle SS in black lacquer... I used waxes with polishing compound in them on it and wore through the original finish. Same thing can happen if you over do it with waxes that have polishing compound. It will take a lot longer than my Chevelle thin lacquer finish did, but it still does more harm, long term, than most of these acids we're discussing.

That's why they said to cover those vulnerable parts with "plastic". Fiberglass resin is just another form of plastic, and as such is usually suitable for storing these acids as containers, or protecting sensitive surfaces from the acids.

I like the muriatic acid because it's cheap... or at least the jug I have was cheap. No telling what prices are doing now.

06-29-2005, 06:35 PM

Do you dilute the muratic acid you get from Lowes/Home Depot? I have used it as well. The oxalic acid wouldn't budge the dried up goo on my Supra. The muratic acid required some gloves, goggles, respirator, and a strong brush.

For the brackish water stains, I used muratic acid with a soft brush full strength and the stains disappear like the oxalic acid treatment. Difference is, about 1/4 the cost of oxalic. I do flush with good clean water thoroughly. Both will take the galvanizing right off a trailer - been there, done that!

Also, don't leave the bucket sitting to long on your concrete with muratic acid. A week later, I had a nice ring eaten into the concrete on my pool deck. The solution dripped down the side from me slopping the acid back and forth with the brush. My wife wasn't to happy :wink:

06-29-2005, 07:00 PM
No dilution... that dramatically reduces the effect. The good part about muriatic acid is it can be neutralized with water... plenty of water. I used a dry mop Swiffer with the regular Swiffer pad on the mop for an applicator. The flimsy pad doesn't last long, but long enough to get the stuff on the hull, and that's all that's needed. It's like spraying 25% Clorox solution on moldy siding, or moldy dock fenders. Just the contact does the work.

Yes, the price is the best part. If they only knew.

When I was in the sheetmetal shop, I used technical strenth hydrochloric acid for flux on galvanized steel. I also used it full strength to clean copper patina off my hands, after a big copper job. Kinda removes the fingerprints too, when you do it, but no other noticable damage... unless I had an open sore. Youch!

I don't recommend using reagent strength hydrochloric acid. We got some of that from the base environmental lab, when our warehouse ran out of technical strength. Holy crap!!! Talk about sliding the galvanizing right off the steal, in a foaming whoosh. Didn't really make such a great flux, either. Not enough of the zinc chloride remained, I think.

07-01-2005, 05:00 PM
Mary Kate On/Off rocks! Just don't get it on anything but the boat. Especially your skin. I normally keep the hose right there to keep things rinsed off. The stuff is no joke, almost like it was made for it.

07-01-2005, 06:21 PM
It also works unbelieveably well on stained "Stainless" railing and the like.

But DO NOT use it anywhere you might have anodized aluminum - it'll strip the anodizing in a trice.


john beck
07-02-2005, 02:36 AM
I've been sitting on the sideline, reading your favorite chemicals for different applications. I truly appreciate your experience and knowledge.
I asked a couple of years ago what to spray on my electrical connections to protect them, but I've forgotten what I used. Please post what to use on the electrical, and what to spray as a protective mist coating on the engine and trans. general surfaces. Thanks again for your expertise. JB

07-02-2005, 08:33 AM
Thank you.

On the electrical connections that bolt to the engine - like grounds and the hot feed to the starter I use plain old marine wheel bearing grease.

For elec connections behind the dash I use CRC H/D Corrosion Inhibiter.

For spraying the engine in general - ??? I never do that. Instead, I paint the entire motor with two coats of Rustoleum (Brush type - NOT out of a spray can).

This is WAY better than Boeshield T9 or Corrosion Block:

john beck
07-03-2005, 12:28 AM
Thanks Lep. I'll pick up the CRC tomarrow. JB