Ham,
ZERO RUST has been used by a number of people as the direct to metal seal coat successfully. A couple of provisos if I may. View ZR as just that, the coat to seal the steel. That is it's design function and that's what it does best.
The ZR needs to go to FULL CURE before doing anything over it as it needs that time to toughen up so it won't react with subsequent products and more importantly all the shrinkage will take place before the finish work over it happens.
Figure 30 days for full cure. And avoid over applcation.....
Now, once all that is done you can do a scuff sand and build your sandable system on top of the ZR. ZR doesn't sand well and will only load paper. Plus, the more you sand into that film, the more you reduce it's ability to protect the substrate. Most of the automotive sandable primers are going to end up on the floor when you block for flat anyway so use it for what it was designed.
Bruce Palmer
Zero Rust
