The VHB tape is expensive, but it really simplifies the
installation of the windows.
I installed 15 windows, this summer, most of them quite large, and it would have been far more difficult without using 3m vhb tape. Another plus, is that it looks really good on the inside of the window, at the frame interface. Nice and even.
You rub the plexi with a red 3m pad before taping. No primer on the plexi.
I used 4991, as it's the thickest "medium density" tape they have. There's a 3mm tape, which is more dense, but not recommended for our purposes, 4991 is 2.3mm thick.
Tape a few spacers/guides around the frame to be sure you get the window in the right position, because once your window touches the tape, it's not going to be easy to move.
We used Dow 795 all round the outside edges. Nice stuff to
work with. I used a fancy, stubby, butter knife to finish the edges. Just wet the knife in a glass of
water, and go at it. When the knife fills with goo, wipe off the 795, re-wet the knife, and keep on trucking.
Cheers.
Paul.