Best sealant for plastic boats

I’d appreciate feedback on your experience of sealants for plastic boats. I need to seal up the rudder mount on one of my kayaks and have used silicone in the past, but I’m finding it has a limited life. Goop apparently does not bond with plastic. What else do you recommend?

Thanks in advance.

3m 5200
best sealant I’ve used. Although silicone sealants have worked OK most of the time.

You may have screwed yourself…
…by using silicone sealer, as it leaves a residue that prevents anything else from bonding. That’s why it should never be used in a kayak. Polyethylene (I’m assuming that’s what you mean by “plastic”) has a somewhat porous surface, so getting the residue off may be difficult. There are “silicone sealer remover” products on the market and I understand that hexane will remove the residue as well. Barring that, clean the area with whatever strong solvent you can find (polyethylene is impervious to solvents), such as acetone, lacquer thinner, MEK, etc. Once you’ve cleaned it, use a product like Lexel, Goop or 3M 5200/4200 as a sealant. Note that the latter two require considerable curing time.

NOTE: If your boat is thermomolded ABS or similar, be very careful with solvents as they can harm the plastic. Consult the manufacturer for their recommendations for removing silicone residue.

Just resealed the foam bulkheads for my friend’s new (to him) 2008 P&H Capella. Nice neat job. I very lightly routed out a groove in the foam so the Lexel seated in there tight to the floor and sides of the kayak.

Lexel is recommended by people like flatpick & salty who post here often enough… good enough for me. And it was easy to use. Lexel adapts very well to the inevitable flex where foam and plastic meet (some measure of flex is natural & good)

G-Flex by West Systems
I’ve had good results with G-Flex on both polyethylene and thermoplastic boats. The key to G-Flex is to follow the prepping instructions very carefully, especially on poly.

However, I’ve never used G-Flex in the presence of silicone residue, so can’t speak to what problems you may have with that.

Call West Systems
They’re known for being super helpful - I’d try calling them to see what they suggest about the silicone.

I’ve seen a few folks praising Lexel, but in this data sheet from the manufacturer, they claim that Lexel is incompatible with Polyethylene:

Anyone experience any problems with Lexel? I don’t want to degrade my boat’s material.

(Necky recommends MA300 by Plexus.)