I had to replace parts of my firewall around where the heater box mounts. My old welder did not do a very good job so, although I have ground the welds down, the surface on the inside of the firewall is a bit rough where the gasket for the heater box sits against it. The original gasket seems to be in pretty good shape. Should I use body filler or just seam sealer to smooth the surface where the gasket sits?

 

Seam Sealer, filler will hold moisture and rot.

 

Thanks.

 

You're going to want to add tack welds to the seems that run along the firewall or any body work may crack. There is a lot of flex there.

 

if you posted some pics so we can see what you are describing we could help lots more..