I've done this basic method dozens of times
this works for deeper scratches the same way it works for dents.
The changes I've made from what this video suggests is to
- generally ditch the razor blade, one little slip and you cut through the super glue and into the actual finish and then you've made things worse, but some times the situation calls for the razor blade approach.
- get hard-backed micromesh pads from grits 1500 to 12000 and use those to make the super glue flat and smooth instead of the razor blade + sand paper.
step 1) mask the dent/scratch with masking tape, leaving about 1cm clearance between the tape and the damage, and have the edge of tape that's facing the damage folded over, so that it's not sticking directly to the finish along that edge (I'll explain why)
step 2) apply the glue to the scratch, ideally get it as flat as possible, that's why you get "super thin", so it won't form such a tall mound of glue. Consider putting the glue in a tiny dish first, then dabbing it on with a toothpick, so that a large amount of it won't pour all over the guitar.
step 3) let it cure for eight hours. It "dries" in five minutes, but it takes a lot longer to "cure", and is not hard enough to be sanded and polished until a few hours have passed.
step 4) lightly sand it down with micromesh pads, starting with 2000 is good, 1500 or less is sort of overkill. The masking tape will make sure that only the super glue and near by finish gets abased by the micromesh pads. Don't press down too hard, or else you will sand the good finish as well as the super glue, you just want to hit the super glue. As the super glue mound gets very flat, it becomes necessary to remove the sand paper since it impedes the clearance, so it's best to use 4000 grit and up at that point to slow things down and prepare for final polishing.
The reason it's important to fold over the masking tape is because, otherwise, tiny abrasive particles can collect at the edge of the masking tape's adhesive and create deep scratches along the edge of the tape as a you work the micromesh pads. Folding the tape both removes the adhesive edge and creates some space for the abrasive particles to travel underneath the tape, where they're less likely to be rubbed and cause scratches you move up to higher grits.
step 5) go over it with grits from 6000 - 12000 to get the scratches out. It's important to go in steps, as removing very low grit scratches with a very high grit pad takes a very long time.
step 6) go over it with buffing compound to make it shiny. Meguiar's Ultra Cut is my favorite, but it might have silicone, so just don't use it on exposed wood, like a rosewood fret board, for example. I've tried silicone-free compounds, but they're not nearly as good.
It's a good idea to clean it with naptha afterwords, or anything as effective, to remove all the extra buffing compound to make sure the scratches are really gone and not just filled in with the waxy compound. I think a lot of people assume a compound is working, and never realize all it's doing is filling the scratches with wax, and not actually removing them.
