Seam Sealer or Silicone to seal panels?

Greetings,
I'm replacing the rear left footwell in my car and I'm getting to the point where I need to seal the seam between the welded panels. I have a couple tubes of 50 yr black silicone caulk that I was thinking about using to seal the joints but was wondering if there was a compelling reason to use the original style seam sealer instead? If so, how is the seam sealer applied to panels that are already welded together? Like drywall mud (w/ flat blade), roof pitch (w/ paint brush) or some other way???

I did find out the hard way that I need to put the silicone in AFTER welding because I added a bead of it in a 3 panel thickness area and it kept catching fire when I welded...

Thanks!

Thanks for the replies. I stopped at the local paint supply store and picked up a tube of 3M seam sealer. I think one tube will do it.

As for the weld through primer, I went to my local welding supply store (AirGas I think) and they didn't have it but said they could get it... Instead of ordering and waiting for it, I've just prepainted my panels and then roughed up or scratched the surface with a sharp object prior to welding so that I could strike an arc. It has worked so far.

Yea, lesson learned with regards to the silicone sealer - it's flammable when wet!!!!

Thanks again!