Here's my touch-up suggestion:
First you need paper matches, a nick sander, touch up paint, clear coat paint (optional), blue masking tape, rubbing alcohol, 3000 grit wet sandpaper and sanding block, small bucket of water, swirl mark remover, and polish/glaze.

First clean the scratch with the rubbing alcohol. Put the blue masking tape around the scratch. Then make a few passes with the nick sander to clean any stuck on crap in the scratch. Wipe down the scratch with the rubbing alcohol again. Apply the paint with the back end of a paper matchstick. Don't brush it in, just kind of let it flow into the scratch. You'll need to build up layers, letting each one dry before you paint in the next one (at least 15 mins). Build up the paint so it's a little above surface level. Once the last coat of paint is dry (if you use clearcoat, it'll be the last coat), then wrap your 3000 grit sandpaper around your sanding block and dip it in water. Sand down the scratch to the surface level, keeping the sandpaper well wet. Once it's dry, use the Swirl mark remover. If that doesn't get rid of the sandpaper marks, you might have to use a Fine Cut Cleaner. Then polish. That's all ( :p ) there is to it.
For the claying, I suggest a Clay Magic bar. Instructions are on it, but basically you spray a lubricant on the car and rub the clay bar across it. Do sections at a time. Afterwards, I like to go over the car with the Meguiars Quick Detailer (that's the best spray bottle I've ever used).

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1984 300TD Turbo 156Kmi
"He who seeks knowledge begins with humbleness" - Buju Banton