What is a good seam sealer equivalent for the trunk?

GeorgeH said
The Wurth stuff is pretty good and the can stuff goes pretty far. Do a little research. Some seam sealers are direct to metal some are not. I prefer the urethane type seam sealers. Something like this. The 2 component sealers are nice, but expensive to use.
Fusor Direct to Metal Sealer/Adhesive

You don't want anything that's going to shrink back or crack and let moisture in. Seal up all the weld seams top and bottom, and cavity wax in the rails if you really want to protect it.

Good advise. Id like to add some info about the DTM Fusor. The directions say you can paint it immediately but I would wait a few hours at least. Make sure you use a paint sealer. Don't just put color on it. Make sure it's the last thing you do before paint so it doesn't get dirty. It can be smoothed out or have a brush look. I use it for everything.