After cleaning/stripping everything out of drip rail channel to bare metal, I would spray the channel with Epoxy primer then come back an fill in with 3m seam sealer firm.  I know, it's very expensive and there's other cheaper brands, but I have come to realization that, it has to be 3m.  Others just don't work as good.
After the seam sealer then you can prime w/ 2k primer filler.

I tried the Eastwood brand and I thought it was worse than the off brand at the auto paint store.  I don't recall the name of the off brand, but it was a silver tube with green lettering.  Again, I have to say there is No substitute for 3M, #8505.  Sorry, I know it's $30 a tube.

I would run a line of masking tape about 1/4 inch away from area you want to seal, then apply even bead all the way thru area you need.  Upon laying it down come back w/ your finger an run it all the way thru the area you sealed in one pass if you can.  This will show a continuous spread and looks clean.  Try to press tighter near the tape line so when you peel the tape it doesn't show a hard line.  Try to practice a little on something til you get a feel for it.  Also try to find as many areas toy want to seal as you can so, you can do them all at the same time.  Such as, door, trunk, jambs or engine bay seams.