Caulking Marelon?

Before launch this spring I want to lay a bead of fresh caulk around a couple of Marelon thru-hulls that will be under water. I have some Boat Life caulk I was going to use but then noticed on the label that it's "not for use on plastic." Any views on whether this caveat applies to Marelon?
Joe

I've only ever used Marelon above water, but I've had good luck bedding it in 4000UV. Bonds fine, seals fine, etc.

Boat life caulk is probably not the best choice. I would go with 4200 if you think you ever want to remove the fitting or 5200 if it's a forever thing.

pete

Life seal OK, Life Caulk not so much.

Laying a bead of caulk around an underwater thru hull with whatever product will do very little. If it is leaking, remove it and rebed it properly. A bead of caulk around the surface will not keep your boat from sinking. You need sealant under the flange of the thru hole. If you rebed it, I'd use 4200 or 5200.

Agreed, just running a bead around the through hull is a time and materials waster.

If it is leaking pull the fitting , clean it and the mounting area properly and reinstall it.

Rule Elastomeric caulk will stick to plastic. It is the only caulk that I know of that will stick. As said above, remove the fitting and clean out all the old caulk and then rebed them.

DDW said:

Laying a bead of caulk around an underwater thru hull with whatever product will do very little. If it is leaking, remove it and rebed it properly.

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Just another voice in agreement with DDW.

I can't speak to the specific caulk and plastic, but if you do call or check the literature, I'd go in with the specifics on Marelon (I think it's something like glass-filled nylon? Not sure though). Otherwise it's like saying "wood" but.... what kind of wood? Teak would be different from pine, etc.

Comodave said:

Rule Elastomeric caulk will stick to plastic. It is the only caulk that I know of that will stick. As said above, remove the fitting and clean out all the old caulk and then rebed them.

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3M 4200 or 5200 will stick tenaciously to Marelon. If you use 5200 you will need to destroy the fitting to remove it. 4200 too, but you may have a chance. Both are perfectly safe for Marelon (which is a glass filled nylon) and are recommended by Forespar, the manufacturer.

DDW said:

3M 4200 or 5200 will stick tenaciously to Marelon. If you use 5200 you will need to destroy the fitting to remove it. 4200 too, but you may have a chance. Both are perfectly safe for Marelon (which is a glass filled nylon) and are recommended by Forespar, the manufacturer.

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If that is what Forespar recommends then go with that. I would go with 4200 so that you will have a chance to get it off in the future if need be.

DDW said:

3M 4200 or 5200 will stick tenaciously to Marelon. If you use 5200 you will need to destroy the fitting to remove it. 4200 too, but you may have a chance. Both are perfectly safe for Marelon (which is a glass filled nylon) and are recommended by Forespar, the manufacturer.

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Gentlemen:
The well-intentioned editorial advice is appreciated, but all I really wanted to know is if anyone knew whether Boat Life is/is not compatible with Marelon seacocks/thru-hulls per the 'plastic' caveat on the BL label. The thru-hulls are not leaking, but the existing caulking is worn away some from the thru-hull flanges in a couple of spots and I want to touch them up for the season. I've used Boat Life in the past for similar situations on bronze fittings with never a problem, just never on Marelon.

I will indeed re-bed them properly (with 4200, I guess) when the time comes (and I have stout wood plugs handy at each hull hole).
Have a good weekend -
Joe

Any caulk you can see worn away from the flanges was doing no good, and will do no good if replaced. The caulk doing the work is under the flange and cannot be seen. If there is a big gap between flange and hull, then I would not put the boat in the water until it is properly fixed. Smearing caulk around and on top of the flanges may be standard practice for an RV roof, but they don't sink if they leak.

As a wrap to this thread, a few days later I was leafing through my latest copy of 'Good Old Boat' magazine and came across this sidebar to an article. It matches the most commonly used sealants against the most common boat materials. Source is attributed to Don Casey, author of the 'This Old Boat' series of books, a pretty reliable authority. But note in the fine print it's lifted from his latest edition in 2009, so may not be up-to-the-minute timely.

Posted for info (hoping the attachment attaches).
Joe