If you are talking about just putting seam sealer over a seam like the door skin to the door frame or quarter panel outer skin over the door post/jamb, use a tube of seam sealer, lay a bead down, then spit (saliva) on your finger and flatten it out. Yes I said spit on the tip of your finger.... Keep the tip of your finger wet with spit and flatten the saler out, if your finger dries out and you get seam sealer stuck to your finger, wipe your finger clean with a paper towel/thinner and spit on it again. Trust me this works.
I did my seams on a prior Mustang project and a friend came over and told me they looked like shi_. So he got the tube out and re-did all of them and I had already painted the jambs. I let him (without flipping out) and he was right, they looked great when he was done and I had repainted again. I work for a paint company, so the time was all I was out.
The other trick, if you don't want it to look 60's or 70's factory but really "precise" or modern is to buy/use seam sealer tape. It comes in a roll and is adhesive on one side, just peel and stick. The stuff sticks like nobody's business. I have yet to use it, but plan to real soon.
For floor pans, trunk floors, undercarraige, under hood, etc--brushable with a short haired brush looks like the factory did it and can make a so-so area look original (take a 1" brush and trim it down). Just be careful as the stuff shrinks and please don't use it to hide real problems.
