We have been using Evercoat Quantum1 filler and their Super Build 4:1 polyester primer with great success. We apply them over an epoxy primer over bare steel.
Both of those products are easy to apply and sand, and they pass the 500 salt spray test, so they are excellent corrosion inhibitors as well.
We have not had any shrinking issues with these.
If you're inclined, here are some videos that cover our bodywork and paint process: