I had said before to use weld threw primer but my body guy that welded the fender togather said the stuff makes the weld come out crappy and he was right. he put seam sealer on the back and then sprayed over that with rubberized under coating.
I will post pics of the pass side when done with it
ready for body filler it is smoth and will not need much. there was a dent in the S15 Fender when i got it i will need to rub it out and then start on the body work