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I have some old dental tools that I use for situations like this where the paint is in a tight corner. However, you need to be very careful not not scratch the surfaces where the o-ring sits.
Ideally, you don’t want any paint in the recess but from what I can see, you have ~ 1/8” of clear surface from the ID of the fluid passage so you probably will be ok. If you’re OCD like me, you could carefully dab in some paint stripper to completely remove all traces of coating but you need to be sure nothing gets into the passage.
In any event, be sure to flush the passages with BrakeKleen before reassembly.