Sorry to be commenting late, but I just read the thread. Sandblasting produces a surface texture (duh) that promotes mechanical adhesion, but the peaks have to be covered to resist corrosion. A thin coating of a primer hasn't got much chance of providing this protection since like most liquids, it pulls away from the peaks and flows to the valleys of the sandblast texture. The primer should be applied thin since it's only purpose is to promotes adhesion and provides the best characteristics of a bonding agent between the metal and the topcoat. The topcoat will have different characteristics since it's purpose is to prevent moisture permeation to the metal surface and retain it's looks. Typically, the primer should be left on only long enough to apply the topcoat. It wasn't designed as an effective topcoat. Applying primer thick enough to provide long term corrosion protection over a sandblasted surface means using it incorrectly. That is the job of the topcoat.
