During the course of use, most bikes acquire dings and nicks in their paint jobs. Enamel paint is a good touchup choice for either steel or aluminium frames. It also works well if you decide you need a different colour to win all those races, in which case you can repaint the entire frame. Touching up a scratch in the frame is far easier, as it doesn't involve stripping the frame of all its parts, but a good touchup still requires a little sanding, cleaning, priming, painting and clear coating.
Remove the wheels from the bike. Use the multitool to remove all bike components from the frame. Spray the lubricating oil into any tight screws or other nonbudging parts to ensure you don't break any hardware as you dismantle the bike. If you have a poor memory, take several detailed photos of your bike before stripping the frame of all its parts. Place parts in marked plastic bags according to where they're used, for example: pedal crank, headset, derailleur.
Use rubbing alcohol on clean rags to wipe down the stripped road bike frame. Clean it thoroughly, removing all grease and dirt.
Sand the frame. This may take quite a bit of time, and while there are alternatives to using sandpaper, such as chemical stripping, sandpaper is not toxic and will not potentially warp the bike frame the way a sandblaster can. Make sure to sand all the paint off the frame.
Wipe the bare metal bike frame clean with a dampened rag.
If there are any little craters or large dings in the frame, fill them in with body filler made for the same type of metal as the bike frame. Follow dry-time instructions on the can.
Use the metal hook to hang the frame. Use one of the holes in the frame to keep the wire from obstructing the paint. Make sure you have the space to spray it from all angles. Also make sure you won't be inadvertently painting anything other than the frame by appropriately protecting the areas around it, if necessary.
Spray the frame with a full coat of primer. Allow it to dry at least 30 minutes, or according to manufacturer's instructions. Repeat for two to three coats.
Once the last coat of primer is dry, add the first coat of colour. Spray the paint evenly and with a light hand. Again, follow the instructions on the can for the best dry time---24 hours is the usual recommendation with enamel spray paint. Repeat this step and add a second coat.
Clear-coat the bike. As with the primer and the paint, keep a light hand so the clear coat is smooth and even---without drips. Allow the clear coat to dry between applications, following the manufacturer's instructions. Some are fast-drying; others can take several days to dry. Apply two to three coats.
Reassemble the bike once the frame is completely dry.
If there are decals on the frame, remove them with rubbing alcohol. If needed, a razorblade can make the job go faster. Work in a well-ventilated area; the sanding, priming, painting and clear coating create a large amount of dust and fumes. Work in a well-lit area. You don't want to see imperfections in the new paint job the first time you're out riding your bike on a sunny day.