That may be a little too fine a paper. When blocking something like this it is very important to CUT it flat, too fine of paper isn't going to do that. That poly putty I am assuming you mean something like "Metal Glaze" or "Glaze coat" by evercoat, a polyester putty. It really is basically regular filler that has had the talc ground finer. So sanding it with the same paper you would a regular filler is fine, so cutting it with 80 is perfectly ok. The thing is you don't want to ask urethane ("filler" primer) primer to do too much. You DO NOT want to ask urethane primer to fill 80 grit scratches. So blocking the poly putty with with 80 is ok as long as you go over it with another coat and then block that with 120 then 180 before you prime. You can prime over the 120 though we have learned in the industry that is pushing it a little. If you apply the primer in thin coats and don't bomb it on it isn't a big issue. But if you apply too heavy a coat it is all full of solvent and will shrink into the 120 scratches later.
But if you block that putty with 80 then apply a few coats of polyester primer then block that down with 120 and 180 before applying a urethane primer over that, it's basically the same as if you had primed right over poly putty, the poly primer really is about the same.
But getting back to the paper grit, doing something like that hood using a little coarser than you would typically use on a filler and first coats of primer is often needed. Because the panel flexes using a paper to CUT it flat is very important.