your right,the best way to do it is all apart,but if you can't you have a few choices.if you jamb it all and assemble it then paint it your going to have some lines,its going to happen.you could go the way lots of people still do and mask the jambs with paper and outline said paper with fineline tape.this leaves a strong edge and people have been doing this for years with so-so results.you could use foam tape in the jambs,you lay this tape in the jamb to seal it to the opposing panel when its closed.it leaves a less than sharp edge.the next one is called Door Aperture tape, the idea is for the foam tape to seal the two panels up to avoid anything blowing out onto the surface.it's avaliable in more than one thickness for diffrent applications.it works very well in most apps.more than one company makes this product but 3m is the most popular.the last peice of advise i can offer is using a product called Smooth Transition Tape by 3m.you would mask the jamb in normal fashion with paper and run this tape on the inside edge like you would of if you went the fine line route.the one hang up i have with this tape is it doesn't take to tight curves very well,and cutting it to reposition it is a pain. these last two options i've spoke of when put to use correctly allow the material(sealer,base,clear)to dust under it to leave a less than hard edge.take a look around online at some forums like Autobody101.com ones like that will get you started.
