I have had the exact same problem and came here to figure out what was wrong and saw your post. I've finally got a swimbait that I'm ready to take to a small production size to sell. The first 4 that I made out of the final mold were meant for my dad and I, 2 for him and 2 for me. We went on vacation last week to Beaver Lake to fish. I finished the baits with 3 medium coats of Etex, applied about every 10 hours or so. The final coat went on Wednesday July 26th. We fished one bait a piece that Sunday the 30th and they looked amazing, I was so proud and my dad was pumped to see my dream finally take off. We were spending the evenings putting together stuff for the new company. I didn't fish that bait again until Wednesday the 2nd. It swam like garbage. It seemed as if the epoxy was softening where the head contacted the body and was catching/sticking causing the bait to barrel roll in the water. Then Saturday the 5th my dad picked up the other bait that I made him and it swam fantastic. But gradually we could see it start to degrade and begin to catch at the joints again. Ultimately all 4 baits will not swim right anymore due to the topcoat.
I ended up catching a monster striper on a Huddleston and took it to my taxidermist when I got home. He recommended trying an automotive clear coat on the swimbaits (that's what he uses on his mounts). I'm just not sure what to try from here. All I know is I'm not letting this die, I'll try anything to get them working again. Should I go to D2T? Automotive? A urethane?
I added a couple of pics of the baits for reference. These were taken pre topcoat.