Sounds like you're worried about lifting issues with basecoat over the old finish. I chemical stripped all the flat panels on my car and blasted in the jambs. It wasn't practical to strip the underside of the deck lid so I scrubbed it with red scotchbrite and comet then used a thinned down epoxy primer as a sealer and painted base-clear over that. So far, I haven't had any lifting issues with the epoxy on the solid old paint. They say laquer really never drys so it's not the best foundation for a nice paint job, but you'll be ok to seal the jambs if you don't want to take it off, sandblasting is a real mess. If you use the chemical be sure and tape up the seams, don't want the stuff leaking out from between the metal after you've got paint on it. Those black 3m waffle wheels will also make short work of the old paint.