Most vehicles you see from the 60's and older will typically show a cracking and separation of the seam sealer to the drip rails. To offer better protection in case this happens in the future, I would recommend a good epoxy primer throughout the drip rails/seam areas (including the undersides you spoke of). Once cured, a light scuff/prep as if you are getting ready for any other paint product, then apply the seam sealer. It tends to set up rather quickly, so a quick dip of your finger in some pre-clean and drag along the surface, it will normally smooth out any imperfections. Additionally, any exposed areas you may see (as in the undersides of the drip rails) you may get a nicer looking job by taping off about 1/8" (more or less) from the seam. Be sure to pull the tape up before it sets too well.
