I got the welting in the first place because I wanted to do the job "right", put it behind me and move on... It doesnt sound like the welting is a good idea, I assume that the 30# felt is thin enough not to exagerate the gap I am already going to have between the fender and the bed? lol at work the tin benders use a gasket material between duct flanges that would seal out water etc and is about the consistency of playdough wonder if something like that would be good?
