[SS427]

Greg, depending on the job I am doing and other circumstances, I either use 3M's seal caulk or acrylic window caulk. The seam caulk is water based and very easy to use. First prime the firewall. Then I simply lay a 1/4 bead along the weld seam and smear with a finger. Once you get the hange of it, it leaves the exact same appearance as the factory after final painting.

On Chevelles, particularly along the upper cowl joint, I use the acrylic caulk and a wooden stir stick with grooves cut in it. It too should be applied after priming and looks factory original when painted over.
Rick