even though it's not cheap. The two forward vent hatches on my boat have leaked. One was resealed by the hatch maker once, the boat yard twice, and me once - all with silicone products, all very carefully done. Each time the leaks reappeared shortly. Then I used the Sika 295UV and primer - this stuff is made for this job by them. It is very different in appearance, handling, smell, and in every other way from any silicone product. Like most PU products, it can be very messy unless you are careful, mask everything, have plenty of rags on hand. As you use it, you will form the impression that if this stuff doesn't work, nothing is ever going to. The adhesion both to the aluminum and acrylic is far stronger than any silicone. That hatch has been perfectly sealed since.

You do have to remove all traces of previous silicone from the surfaces, and that is time consuming. I used a silicone removing solvent and a lot of time and elbow grease. You also want to space the acrylic away from the aluminum by some small amount - if the adhesive is too thin, there is not enough elasticity to accommodate the considerable thermal expansion differences. This is true whether you use PU or silicone. You can do this with little bits of rubber or plastic that will remain imbedded in the sealant.