It depends on whether you are rebedding the aluminum frame or resealing the glazing to the frame. I rebedded my hatches in 2008 with 4000 UV. That is between the aluminum frame and the fiberglass
deck. So far, no leaks. Now I need to go find some wood to knock on.... this fiberglass
just won't do. I haven't seen any discoloration or adhesion failures, then again the frames are screwed down so the 4000 is only acting like a gasket
used astm 920 sealant and it wasn't worth what he paid for it (which wasn't much). That standard is designed for cars where it apparently works quite well but it doesn't seem to hold up on fiberglass. In any case, I've been rebedding all of my aluminum flanges with 4000 and am very happy with everything but the price
Resealing the polycarbonate/acrylic is a different animal. Silicone does not adhere to metal very well without a primer. Unfortunatey the primers cost $50 acan and you only need a few drops. Once Silicone is used it's almost impossible to ever get a new seal to adhere. I have had some luck cleaning the old surface with 3m general purpose adhesive
remover to get the old silicone residue off.