CD 28 Skylight

[Jac Clement]

I have to replace my saloon skylight. I rebeded it last fall with Boatcalk and when I removed my boat cover recently I discovered multiple cracks around half of the screws. I consulted a local plastic company and he didn't think it was the Boatcalk but rather I probably tightened the screws too tight and the cold Wisconsin winter probably expanded the acrylic enough to crack around the screw fasteners. He said the skylight is acrylic and said polycarbinate would be stronger and wouldn't crack like acrylic. I contacted another plastic supplier who could sell me a gray tinted opaque Polycarbinate sheet measuring 3/8" thick by 24"x48" for $133.50 (thats the smallest sheet he had). I only need 24"x24" so I could make 2 if I need a backup. My question(s) to the board is: Do I need to stick with Acrylic (Lexan) because it has better UV protection/resistance? Will polycarbinate hold up better to UV rays. Or was it the boatcalk that caused the cracking/or was it because the original acrylic skylight was just old (40 years). Do I need a gray tint, like the original, so it is not to bright in the saloon. Is the $133 a reasonable price for the polycabinate sheet? Any advice or suggestions would be appreciated.

[John Ring]

Those bugle head screws make great wedges as you tighten them down, probably where the cracks came from.

Most modern lenses are acrylic, but I just reglazed my fwd hatch, and replaced my skylight with tinted and UV stabilized polycarbonate called Makrolon. An eBay seller called "popdisplays" sells small pieces and will cut to your finished size for free when you buy his pieces. The 1/2" material I used for my skylight, cut to my finish size, was around $65. I rounded out the corners in a band saw, used the old lens as a template for the holes (and skipped the countersink as I wasn't going to use bugle head screws again), then ran a nice bead around the whole edge with a router. It came out pretty nice.

The old skylight's silicone sealant came off the cabin top with "De-Solv-It" and a scraper. I dumped the old bugle head screws in favor of flat tops, so the wedge effect is gone, and all screws tightened down nicely with no cracks. I bedded the new skylight down with DAP 3.0, as the screws make the actual mechanical fastening.

On the forward hatch (3/16" polycarbonate), I cleaned, primed, & painted the upper frame, and installed a new gasket. In this application, the sealant is actually the mechanical bond holding the polycarbonate lens in place. As such, you need special polycarbonate adhesives. I tested, then used bronze colored Dow 795, and found it holds polycarbonate really well.

The skylight and fwd hatch look new!

Good Luck!

John Ring
CD28 #241 Tantalus
P.S. For adhesion testing, I glued two piece sets of scrap polycarbonate together with 3M's 4200, DAP 3.0, and another set with Dow 795. The 4200 held polycarbonate about as well as latex paint would (came right apart), the DAP3.0 did pretty well, and the Dow 795 was really well attached.

[Ron M.]

I replaced both hatch lenses 5 years ago. had same problem with cracks around screws. Pompanet,(sp) the maker of the hatch recommended using sealant only - no screws. Their advice was to use GE Ultra-Glaze structural glazing sealant, #ssg4000 - it's a modified silicone. A good bond although it's black and messy.
tape off areas where you don't want it. I couldn't find it anywhere so I bought it from them - 25.00 for a 10 oz. tube. I haven't had an issue.......knock on wood.

[Jac Clement]

Thanks for your suggestions, it's a big help!