Steve;
PPG, MP-170 is a Two Part Catalized primer. My PPG Supplier suggested its use on a bare block prior to top coat. I have 3 engines now shot with PPG, MP-170 and then Top Coat with no effect of heat seen. I also use the PPG, MP-170 as base primer on my trucks. Its tough stuff. This primer has a 3 day top coat window or it must be lightly sanded and more primer shot. Shooting topcoat should not be a problem in a 3 day window. However the window must be adhered to. I only use it on Blocks, heads and all bolt on accessories. Exhaust manifold just gets high temp BBQ Black.
Paul