I was a ASE Master Auto Body tech (gave up renewal in
2002). Not bragging, just tossing some credibility.
Urethane adhesives should always have a primer matched
to that particular adhesive ie Sika, 3M, etc.
In addition with Plexy I'd take 600 grit sandpaper and lightly
rough up the bond area. Use masking tape to avoid damaging
adjoining Plexy. After sanding, use a good solvent like 99%
alcohol to wipe the bond area (careful and use only a bit).
Allow to dry completely and then use your primer, then urethane.