I have heat treated the membrane on my pre-PX panels.
My earlier panels had the Spandex grille material so
heat treating the membrane without removing/pulling it back is
easy, just be careful not to overheat the material because it can melt, ask me how I know.;))
If the panels have the double knit material as current production panels currently use then I would pull the material back as Tom recommended.
In my case I needed to treat the entire panel(U1) as the membrane had slackened overall.
As to a suitable heat gun, I would suggest a Wagner Furno 500
heat gun as it offers 12 heat settings. It allows you to dial just the amount of heat necessary to do the job.
Couple of pointers:
1. Try a low heat setting first, don’t want to melt the membrane.
2. Keep the heat gun moving and probably at a distance of +/-
4-5 inches from the membrane.
3. Take your time, just be conservative with the amount of heat used.
Good Luck, let us know what happens.