Why not use Gelcoat (flowcoat really)? You don't really need to deepen the scratches for that, just cleaning them up is enough, although a fine Dremel bit to make it a little wider if it's a very narrow scratch might help. Definitely do degrease with acetone and then brush on some.
But also keep in mind it's the bottom and doesn't have to be perfect. And since it will be painted over anyways, you can also use (thickened) epoxy, which will definitely never fall off.