I have had good service from the UPOL "tiger seal" it is an adhesive seam sealer that is easy to work with and so far has served me and my customer cars well for several years now. I usually apply it to the drip rails in two coats, as putting seam sealer thick on the first coat seems to really affect dry time (if it ever sets up at all). I put a small bead in with the caulking gun, then using a spreader or whatever I can get down in there (usually my finger

) spread it smooth and let it dry at least 24 hours, then apply another small bead and smooth it again to build the factory looking thickness. So far I have not had it crack, shrink, or separate. I have been using it for almost 5 years now, after the last tube of 3M literally cracked and fell off the car prior to paint. If you decide to put any brand very thick (more than 1/8"), you should give it a few days to dry before top coating it with anything to ensure a good dry. I have heard good things about the 2 part fuzor sealers as well, but have never used them.
Kelly