Since I did a section at a time over the past year, I'd simply used Rustoleum rattle can filler primer then wet sanded as needed.
Since you have been using this the entire time, you will need to figure out what will work over that stuff. If you spray something incompatible you will have to start all over again. I was always told to pick a system from one company and stick with it throughout. I went with Nason since it was available at the local auto paint supply shop.
