All fillers should go over epoxy IMO. And NEVER apply fillers over an open seam. Weld that sail panel seam up solid if you're going to use filler. And if you ever use body solder on an area like this weld it solid or make sure the solder flows through the seam so there's no chance of trapped acid-I learned the hard way on that, also explains why some factory roof joints failed early and others lasted a long time-all depends on who did the solder job and if they flowed the solder through the joint.