Hi Bill and Charlie

 

Sikaflex is a good caulking and it does come in white and black or mahogany and white in the case of Sika 240.  For plexi, I have used Life Seal by "Boatlife" which is designed to bond to plexiglas and other plastics including fiberglass.  I used this for my boat's windows and for the kelp windows (1" thick plexiglas windows at the leading and trailing edge of the keel).  As boat's go, that is about a critical a bond as they come.  Should that fail, the boat would sink in a matter of minutes.  The trick I did was make a deeply textured bond area that held in both an adhesive and a mechanical fashion.  Not to be cautious or anything but I also put two set screws in it at each end so that even were a bond to fail, I still had it held in place mechanically.

 

I did testing of the Life Seal, vs. 3M 5200, Life Caulk and silicone.  The Life Seal bonded the best.  And it was tested against both gloss and 120 grit sanded Acrylic and Polycarbonate. The sanded material performed much better than the glossy (but clean) plastics.  To be fair, I did not try the Sikaflex but a good friend who has built many boats claims it bonds well to plexi.  He too has used it for below the waterline kelp windows.  In testing I did years ago as a boat builder, we found that the plexiglas had better bonds using the primer before placing the windows and yes, Sika has primer.  Regardless of what you use, one thing NOT to do is use acetone to clean plexiglass!  It can cause lots of problems later.  Again, I did not try this but it is based on some horror stories I heard about cracking later in life.  And as you probably already know, if you have to drill through it, you need a plastic bit that has a different pitch on the bit and also ideally, you should anneal the plastic after machining to relieve internal stress.  TAP plastics sells them as do other plastics supply places.

 

I hope this helps!

 

Dan Newland