To paint or not to paint over 3m 5200

[BurnabyRocket61]

Hello, first time poster, long time reader. I'm replacing the raised bow deck in my 16 ft welded aluminum boat with marine grade plywood. I've epoxied and 2 part primed the plywood, and I've vineylux washed and 2 part primed the aluminum sides. This leaves a small gap between the two that I plan on filling with 5200. Do I caulk, then paint over everything with 2 part Interlux? which might crack because of the possible movement from expansion between the two surfaces? Or should I paint then caulk which might leave me a possible inconsistent finish between the two surfaces? Thanks.

[PAR]

Caulk first then paint. You want the caulk to bond to the substrates, not the paint. Tape off each side of the seam carefully if you want a nice neat job of it. You can paint up to the caulk or over the caulk. If you have much movement the paint is going to crack and show a line eventually, so maybe leaving the caulk unpainted will look better then a crack in the paint. White 5200 will yellow slightly with age. 3M 4000 UV will resist yellowing better, but has about half the tensile and elongation strength of 5200.

3m 5200 is a tenacious adhesive/sealant, heavy on the adhesive side. If you're just sealing the edge and expect the fasteners to do the holding, 3M 101 or BoatLife caulk may be better choices for you.

[BurnabyRocket61]

Thanks for the advise Par. Further to this, do you know if it's possible to first fill with 5200 and then top off with 4000, ( I would like the strength of 5200 and the uv protection of the 4000), and if so how thick should the 4000 be and should I top off right away or wait until the 5200 partly cures or even fully cures. And hey how bout those Panthers.

Thanks, and remember to keep your head up and your stick on the ice

[PAR]

I would think partly filling a seam with 5200 would be a difficult bit of business at best. It's gooey, sticky stuff and being neat about it, is a oxymoron. I would think 4000 UV would stick to 5200, but again it's a lot more effort then I'd want to try. I'd let the 5200 fully cure first.

I think you're going to have some differences in the texture, surface irregularities, smoothness, etc., between the pieces you're putting this seam in. It would be nice if you could have a flawless seam, but on the other hand, you could make the seam clearly visible and eliminate much of your concerns. By this I mean, let the seam stand proud or slightly recessed. It's there, no one's kidding any one, so let your handiwork show with a nicely caulked seam, maybe in a contrasting color.