SPI epoxy, and most others will fill 120 grit scratches easily. Period!
I have filled 80 grit scratches with SPI epoxy many times
and never had a problem. (2 full strength wet coats)
The key is to have it cured enough before sanding that the
primer doesn't shrink after the sanding and the scratches show again.
I always let it cure 2 days out in the sun before sanding.
As long as you allow enough time after spraying for it
to cure before sanding you're fine.
The longer the better, everything shrinks, even epoxy.