Not all the models I have are unpainted... some are ones that I inherited that are badly painted.
This Demolisher is a good example.
I didn't try and strip the paint.
I gave it a matt black coat of spraypaint, then a khaki drab coat from the airbrush.
The cammo stripes are khaki then lightened with bleached bone.
The entire tank was given a bleached bone drybrush.
The entire painting time for this (excluding drying time) was about half an hour. I love my new airbrush!