You can buy liquid latex at a hobby or craft store and comes in a plastic bottle (also called liquid masking or latex masking). It is the consistency of paste, but you can use a small brush etc. to apply it and dries quickly (it just stinks because of the ammonia).
What I did was to brush on some latex in the same shape as the damage in the reference pics for each color I was doing (it dries in a few min. if thin and longer if thickly applied) then painted it.
After the paint dried, I refered back to those pics and scratched/pulled the latex in those areas and quickly rubbed it with a paper towel to get what I missed.
Be careful not to drag the paper towel or peel the latex in one particular direction, I did this and it rolled off some of the paint and gave me weathering that I didn't want :cry.
Hope that gave you some idea of what this stuff can do.