Well the finger nail test is okay but I wouldn't trust it. I did a truck (haven't posted it yet) but it was scratched up like crazy, they weren't deep scratches (I did the finger nail test) and my nail didn't get caugh so I used Optimum Compound with orange Lake Country pad and rotary, the rotary was operating at 1500rpm. After a few pases, the scratch was still there. I then proceeded to wet sanding. I wet sanded and the scratch still there, after a few passes of wet sanding and I could still see the scratch I gave up and went over with Optimum Polish and Orange Lake Country pad to remove the wet sanding.
The point is that some scratches you won't be able to remove even with wet sanding, some scratches may not look deep or feel deep for that mater but you just wont be able to remove them completely. The idea with of being able to remove scratches with the rotary is true for majority of the time but some scratches require a very long process. I came to realise this. To get a vehicle flawless and that has some bad swirls/scratches you will probably require a five step process and most likely some wool pads to do some serious work. But, keep in mind that even then some scratches may not be removed.
Well that's just what I have found out...don't get me wrong with the rotary and faom pads you can correct lots of scratches but not all scraches. Well like I said just my opinion....I hope this helps. On my own vehicles I'm going to do some tuch up paint to remove deeper scratches (the Volvo) to get the vehicle looking perfect.
