Firewall seam sealer.

[Squareforceone]

hey, i'm in the middle of swapping engines in my 84 and want to clean up the firewall a bit. I am however not stripping everything off it just some of the easier things and touching up with some rattle cans. I do however have some minor rust that i want to address while the engine is out. I think i need to strip a lot of the old sealer as i feel it's probably contributing to the rust issue.. I'm wodering what's the best way to strip it wire brush on a grinder? I'm also wondering what would make a good replacement seal on the cheap. I'm on a budget right now so i'm not looking for expensive eastwood seam sealer or anything.