There is a brushable paint mask solution, I think it is modified latex, I have seen it at true enthusiast hobby supply stores. I recall the name Mask-it.
If you look at what some of the pro bike painters use, it's tape, namely 3M fineline mask tape, terrible grey green color. Not a crepe tape. Automotive pro paint houses carry it.
I use that and scalpel blades, sharper than X-acto hobby blades. Also use a magnifying lens lamp ( showing my age with that)