Gelcoat adhesion to canopy problem

Help, I have a chronic problem with my canopy. Years ago the
plexiglass was replaced on my canopy. Since I have owned my glider
the "white painted edge" of the canopy has been flaking off. The
gelcoat seems to just lift off and flake away, leaving a ragged edge.
Some areas have the gel lift up into the slipstream probably not
helping the airflow.

Every year or two I mask off the edge and hand paint layers of gelcoat
back, then sand smooth and even again. Then a few short months later,
time to do it all over again.
My new gel seems to stick better than the older stuff but even some of
what I put there has come off. This seems to be a problem on the
overpainted plexiglass transition area and not the frame portion.

I think part of the problem is that the smooth plexiglass does not
make a good adhesion to the gel. I also assume that there may be
differential expansion in hot/cold cycles, sun that may make the gel
and underlying plex to work apart?

So before I put on more gel, I'd love to hear some suggestions on
surface preparation or materials etc.
I have Simtec Prestec 2781 gelcoat.

I have been hesitant to really scratch/roughen up the surface of the
plexiglas, the shop before me did not do it and I would be afraid of
creating any kind of stress risers but maybe I'm just paranoid.

Should I use some kind of primer?

Chris