Dont use a brush....
Use a piece of card which has been cut to a thin point.
It is difficult to explain....I have a "mer" sctach and chip repair kit. It comes with what looks like a book of matches, but without the heads on the matches!
Anyway, you just tear out a "match" and angle cut the end using sharp scissors, so you are left with a really fine point, almost like a needle. You then use this fine card point to apply the paint to the chip - it gives you incredible accuracy. It is the recommended method used by pro-valeter types.

As I said, its difficult to explain, but hopefully you can see what I mean?
Give it a go!
Andy