Before the motor'n'stuff go into the chassis, it needs paint.
That's a simple enough job - give the metal another good clean and then rinse it thoroughly under running water. Then let it dry in a nice warm spot. This is a stage you don't want to rush as tiny drops of water can hang around for a long while. A blast from the hairdrier helps, but I prefer to leave it 24 hours to be sure.
Next, a light dusting with etching primer. I'm using UPol from Halfords at the moment and it seems to work OK. The cheapskate in me remembers that I was always told a light dusting, rather than a heavy coat.
Finally, I brush paint Revell No.9 everywhere. I could spray, but it's easier to brush and in my mind, the adhesion is better. A couple of coats on the sides seem sensible.
Finally, the bearings and brake hanger wires are cleaned up with some scraping. I'll need to solder the brakes later, and you really don't want paint in the moving surfaces.
Then, let it all harden again before moving on. Easy, if you are too busy to work on the model every evening.