Which grit for prep to fill holes then after for paint prep............

Can you elaborate a little? Holes in what? Rust holes? If these holes require welding make sure you get the paint back far enough where it doesn't get into the weld.

Fiberglass hood and rivet holes.............

Yeah, I just saw your other post. Don't go coarser than about 280. Use 320 if it will do the job. Have patience Danielson.

We treat glass pretty similar to any other media when sanding. Start coarse and gradually get finer. The finer you end up with the better the outcome will be in the end. Rarely at the body shop do we go finer than 320 on rental car repair work, they just don't pay enough to spend a ton of time. Good primer surfacer will fill in a lot. When we paint cars depending on cost we have gone much much finer, just more time and money.

You've seen my car Al, we can't make the hood look better than the rest of the car................lol

dom, you better not ask me. my method is grind with 24 grit to bare glass, slather on bondo, grind off bondo with 40 grit feathered with 80, then prime and paint. [with a rattle can] um, i don't own, nor come within 50ft. of show cars.......

Not my strength either but will hopefully get it done today.......

240, then 400, then a high build primer will work OK.

Just fill the holes with silicone.

36 or 40 grit to start, lay your fiberglass or kitty hair, cut it down with some 80, fill/refill as needed, then 100, prime with high build urethane primer, sand again with 100 then spread with polyester glaze, sand it with 220 and prime again with finish coat, spray mist with spray bomb for guide coat, then finish sand with 320 to 500 depends upon paint being used and painter preference.

This is close to my suggestion, except for the numbers . A fresh 36 will eat up FG really fast, too fast, unless the objective is needing a bigger hole. 80 would be my start, and 500 is for 5'/0 mph car wearing glasses