I've done touch up many a times for my buddies before a car show- most were on the bumper or the hood, but they are the same principle- two factors make up the noticable difference- texture and color.
If its black, the color part is gonna be the easiest- The texture is gonna be your enemy. If the paint was clear coated, it helps a lot. If it wasnt, your gonna be in a bit more hassle. (most newer trucks have clear coat, so.. )
Anyways, Use masking tape, tape up the area along side the scratch. Leave around 2cm on each side of the scratch.
Gently use 600 grit sand paper and try to smooth out the edges of the scratch. (dont forget to wet the sand paper). Dont worry too much about hazing on the exposed 2cm area. This will get polished out.
After fully drying that area (even using a little brake cleaner on a rag to remove all particles), use the touch up paint stick, and try to fill in the gauged area. Do it in small layers, and wait for each layer to dry. You can speed it up with a heat lamp if you have one.
The final layer should be slightly humped as compared to the original paint. After letting this final coat dry, use a wet 800-1000 grit to gently (no pressure from the hand) smooth it all out so that the filled in layer is the same height as the old layer. When its about passible for height, step up to 1500, and then 2000. The paitn should be ultra smooth at this point.
Remove the masked up area. Use the clear coat spray, practice a few times on cardboard to minimize overspray and what not. Try to get a nice even coat
A light application should do it.
Once that layer is dry, you can give it a once over, and see how it is. Careful about overspray of the clearcoat-, thats the hardest part.
