There are scratches, and there are scratches. Some scratches are very light, while others are deep. The same thing is true with any surface imperfection, such as RyRy210's water spots.
The basic principle in removing scratches is to start out with the mildest product; if it doesn't work, use something a little stronger; if that doesn't work, use something that's stronger still. However, the stronger the product, the better the chance you will be removing paint, and when you start removing paint, you may damage the finish. OTOH if you have a scratch that is so deep that you think you are going to need to repaint the panel anyway, stronger products may present less of a risk, just because they can be used as a last resort before taking it to a body shop for repainting.
Zymol HD-Cleanse is very mild; I use it routinely on my cars. I can go over the entire car with it, and it removes oxidation and
light scratches and swirl marks and tar marks, yet doesn't remove any paint that's visible on the towel.
If there is a scratch that is deep enough to feel with your fingernail, HD-Cleanse definitely isn't going to remove it, so you'll want to try something stronger. I haven't used the 3M Scratch Remover that Jimbo mentions - in fact, I can't find a product with that name on the
3M website, although 3M has a variety of glazes and compounds shown there - but I have used the
Meguiars #9 Swirl Remover 2.0, which I suspect is similar to what Jimbo is referring to. It's abrasive - sufficiently abrasive that it will wear down the paint ever so slightly (you
can see it on your towel), but fine enough to leave the shine on the finish.
If the Meguiars or 3M aren't strong enough, the next step is to try something even stronger - either a rubbing compound or a very fine sandpaper. At this point, you are going to be taking off quite a bit of paint and when you're done you will need to buff the paint down with the Meguiars #9 or very fine sandpaper to restore the smooth finish after you're done. But, like Jimbo, at this point I would not want to use compounds or sandpaper myself; I would leave it to the services of a professional detailer or body shop.
[This message has been edited by nsxtasy (edited 23 September 2002).]