I'm having a hard time keeping rust from showing on my manifolds after I've coated them. I used VHT 1500 degree cast iron manifold coating, and it looks great for a while, then rust starts popping up again. I've blasted the manifolds and coated them right away, and that still doesn't seem to do the trick. Getting tired of yanking manifolds off. Anyone have any tips on how to keep that nice clean cast iron look? What's worked for you?