When DPLF & DPLF-401 activator is used over bare metal is the best option. It is designed to have a 7 day open period to apply fillers or paint over before having to sand & recoat. That is the best method to apply fillers over the DPLF after the appropriate wait time (24hrs if I remember correctly). Everything is designed to chemically bond. I have never had any issues or gummy when following PPG’s instructions. After the initial coat of the bare metal you can follow sequential top coats of DPLF using the DPLF-402 activator as it’s more of a sealer coat. Good luck on your projects.