I've been doing a method where I place a small blob in a more open area and use the very tip of the rubber brush to just break the surface of the blob and then pull it into the smaller lines. Very small thin scratches you can use toothpicks, straightened staples, calligraphy nibs, atoms or anything else small and thin you can find.
For some of the super fine detail, consider going topical. I found you can have a bit more control if the mark you're making is thin enough and you're not getting the level of detail you want from the masking fluid.
Like others have said, I've also had good luck with the micro brush for some parts.
thanks for the advice. ive just ordered some 1.5mm microbrushes. ive decided to do a practice panel on some plastic before i touch my helmet. so i have the chance to experiment and see what works and what doesnt.
This may be a late response but I ran into the same issue with trying to get into the fine details and none of the local hobby or craft stores had what i needed but then my wife came to the rescue she told me of a silicone tool she used for her make-up applications. Its got a fine tip as a number 2 pencil would have and clean up is easy as can be just look in most eyeliner make-up brush areas and they should have them in most stores.