Joe,
You let the Zero Rust shrink into the irregularities in the surface of the steel and it then becomes the surface. Your automotive system will have a high build sandable primer surfacer that is the product that will be block sanded to get rid of highs and lows. Just a light scuff of the ZR will ge the primer stuck.
Call me if you'd like and I'll give you a couple of good sources of info on all this. 800-725-4960
Bruce Palmer
[email protected]
