Schodlatz and CGS are the experts here, but my experience is that if you just build up the scratch with clear finish, you will always be able to see the scratch through the new finish because the original poly goes slightly white along the scratched edge. Solution is to brush/wipe just a bit of stain in the appropriate shade on the scratch to blend the color, let it dry, then start building up the clear finish. Gloss lacquer will build faster than poly (because it dries faster so you can apply coats at shorter intervals), and when you go to polish it out, it's just a bit softer than the surrounding poly so you can cut it flat with very fine automotive wet sandpaper (1500-3000 grit) without disturbing the poly much. Then polish.
