There is really no intricate secret. Just patience and a pair of steady hands to hand paint with a fine tip brush.
It is best to have a picture of the model. As detailed as possible. There were times I cut off part of the body by mistake while removing the "supports". i.e. a finger from Porky.
Different sizes of paint brushes are necessary, especially the fine tip ones.
A long needle-pointed side cutter will be very useful. I don't have one and it is problematic trying to remove "supports" from a narrow area.
Sharp Exacto model knife is a must.
File bars and sanding paper to smooth out the rough edges and cut area.
As to painting, I am using Testor enamel paint. The kind I used while I was a model car hobbyist, some 6 years ago. I have experimented on acrylic paint but it lacks the "shine/lustre". Applying clear coat over the acrylic paint is not always satisfactory.
Spray paint is useful when large part of the body is of the same colour. Save the time to hand paint. Besides, spray painting result is always far more superior than hand painting, provided one knows how to spray paint. Spray paint the entire body first, then hand paint the parts that need different colours.
Using different colour filament is a logical alternative. i.e. using black filament for Mickey, or white filament for Star Wars Storm trooper.
Don't get obsessed with spray painting. Only when necessary. Be aware of the danger of "over spraying" and ruining a perfectly printed model.
Sometimes a damaged model can be salvaged by using Polyfilla. Example, once while trying to cut off the left-over supports, I cut off a tiny part of the body. I applied the polyfill to the crack, sanded it after dried, repainted the affected area, and the model was good to go.
I use Gorilla super glue.
P.S. I don't use primer. I don't see the necessity.