Gotta disagree with above.
Provided the car hasn't been excessively "cut" previously then you will do no harm with a GENTLE T-cut.
However better to use Farclea G3 - it's a much better product (in my opinion)....
Some simple rules to remember.
Run your nail across the scratch, if you can distinctly feel it, no amount of T-cut will sort it - but it will make it better. However don't keep cutting till it's ok, you'll take the paint off!
If you can't feel it with your nail it will almost certainly cut out and fairly easily.
If cutting by hand, don't be to aggressive to start with, a little on a cloth, rub the affected areas gently, frequently wipe clean and take a look at how you are doing. Don't over-cut, better to get it pretty good, then move to a finer compound (G1 or similar).
If machine cutting don't use G3 unless you know exactly what you are doing - you'll cut the paint off - use G1. Do NOT use a power drill unless again you are a pro (in which case you wouldn't use a drill anyway) - you'll "burn" the paint - use a proper polisher (1400rpm or so vs 15000 rpm or so with a drill) and a good quality foam pad, keeping the surface wet and lubricated. (most put a bit of fairy into the water, then spray onto panel frequently).
Slowly slowly is the key, cleaning regularly to see how it's going - don't try and do it all in 2 mins or you'll damage the paint.
