Silicon adhesion question

[Paul Denison]

Due to their age and cost to repair and leaking, I am covering my light niches in the pool. I want to sandwich Lexan between the fiberglass wall and the light niche ring. I ground down the bronze ring to accept a round lexan window so that I now have two flat surfaces. The ring will take screws into a backing ring behind the fiberglass wall. I am looking for the best sealant to use in a chlorine environment. There will be sealant between the ring and Lexan and between the Lexan and fiberglass. I am thinking silicon or a polyurethane.

I was concerned about adhesion to the Lexan.

Thanks in advance.

[P.I. Stazzer-Newt]

Lexan, and the other high strength polycarbonates, used to have a really bad reputation with solvents...

[TC]

If you can use the silicone more as a gasket material, and less as a sealant, it should serve well. Put the silicone where you need it, let it set up, then tighten the screws. Learned this trick from Don Casey's book "This Old Boat." Never used it underwater but assuming that you're not asking the sealant to make up for some other weak link, I should think it would work. Lexan and silicone are compatible, don't know about polyurethane.
Tim

[Paul Denison]

Do you mean solvents in the sealant or the pool? What would you recommend as an alternative?

[Paul Denison]

If you can use the silicone more as a gasket material, and less as a sealant, it should serve well. Put the silicone where you need it, let it set up, then tighten the screws. Learned this trick from Don Casey's book "This Old Boat." Never used it underwater but assuming that you're not asking the sealant to make up for some other weak link, I should think it would work. Lexan and silicone are compatible, don't know about polyurethane.
Tim

Thanks, now I recall that trick from Casey's book.

[Paul Denison]

Thought I'd move this from the bilge so more would see it.

[TMny]

Lexan is a GE trademark. GE still provides telephone customer support (???!!!???) from a number on the silicone caulking cartridges. It might be worth giving them a phone call.

I'm very new to the urethane (3M5200) caulking, but i thought it was generally superior in all respects except temperature resistance, and curing rate , as contrasted with silicone.

[Mike Vogdes]

Another vote for 5200...

[pipefitter]

I dont know how silicone sticks to lexan but it sure adheres to gelcoat. Once cured you will find it takes a razor blade to get it off and you will still see the residue. Also,in order to get it free after the fact,you will need to cut it,provided that the gelcoat wasn't oxydized(chalked) to start with.Is what alot of modern glass boat manufacturers are using to bed their hardware with these days.

[TMny]

>>Is what alot of modern glass boat manufacturers are using to bed their hardware with these days.

Thanks , pipefitter. I've used a lot of silicone, and noticed that it does seem to bond very well to gelcoat ... and leave the residue you speak of. The manager of our local boatyard says he restricts 5200 deployments to those projects he hopes never to want to undo !

Can heat be used to soften 5200 (as with epoxy) ?

[Mike Vogdes]

Its tenacious but not at all unremovable, seems like everytime someone brings up the use of 5200, someone else will chime in on the difficulty of the removal. If your looking to bond plastic to fiberglass and don't want to be bothered with it leaking then 5200 is a good way to go.