I generally stay away from repairing rust but this customer insisted this box be repaired. This box had the normal rust on the top rail with holes. We got creative and shaved the seam/body line. i sandblasted the rust and sunk the holes and filled the entire seam with All-metal thick. This created a thick barrier in which i feel the rust should not come through for years. the problem is in the sun the fillers seam to be swelling not sand scratches or shrinking but causing the body work to look rough when it is in the sun for a bit (the truck is black). When it cools it seams to relax. Ive been in the collision and autobody industry for 15 years and never seen this happen. many many hours went into this truck putting a show car finish on to it. i haven't delivered it to the customer yet and fear he will see this and want answers. my question is will this continue to happen? how is this avoided? And does any one think the filler will fail (not from rust but from thick filler)?