If you are going to put body filler on it you do not put anything on the metal. The filler goes onbare metal. Sand it with 36 grit sandpaper. Get you filler done and once its smooth and blocked off you put etching primer on any bare metal, a thin coat. Then you can use an epoxy primer and cover the whole work area with 2 coats. Then block sand it to look for low or high spots. Then put glasing putty on any small low spots. Prime it, block sand and repeat process till everything smooth.
You can't lose if you put God first!
1989 LX, 306 with TF top end kit, Anderson N-21 cam, TKO 600 trans with steel splatter shield,full Maximum Motorsports Mustang maximum Grip box, Detroit locker rear end with 31 spline Moser axles. Staggered wheels with Nitto 555 315/35/17 out back.