I use a product from time to time called "Rust Defender". This stuff will really build fast and it is a two part system which requires a catalyst to set up. You do have to use a larger air cap or fluid tip to get proper sray out but it lays down well. Block sanding is easy and I have nevr had any problems as a substrate. If you don't need that much build up the good old PPG K-36 is about my favorite. It also is a two part product that gives excellent performance. The nice thing about K-36 is the fact that it can be tinted using PPG "Radiance" dye to be used as a sealer coat before final color coats. There are others out there but these have never failed me. Good Luck! JC.