Tried to wash it with water, detergent, and PCB cleaner, still a lot of gunk...

Same here brother

turpentine would work best, but a cheapish brush for onetime use is also a clean solution

I never bother. I just brush the hard bristles against my finger to break the bristles apart when I am ready to use it again.

I don't have this issue because I don't actually bother conformal coating the electronics in my quads and just make do with what they (might have) applied at the factory. Not had any issues so far by not doing so (knock on wood), but I try not to fly if it's damp anyway.

+1 i hide this work, the quad which was going into the river didnt die- it was just clean xD indont have a reason for the work :-)

TBH I think a lot of the issues stem from other problems and not from a short circuit between contacts due to moisture getting on the boards. Conformal coating is probably a bit of a placebo effective for a lot of people where they believe it is actually helping. It also creates a horrible looking mess on the board if you ever need to remove it or burn through it to re-solder anything. As mentioned, plenty of people have completely drowned their quads in a lake / river with no conformal coating on and they still work again after having been properly dried out. I have just decided to keep the boards in my quads "clean" and take my chances.

I don't use conformal coating either. I tried it once, but was not happy with it.
Yeah, I try not to fly in wet conditions.
I just take my chances.

I do, however, use conformal coating as a spot insulator for certain repairs.

The brush is in cap, I just use it as it is...I don't clean it or mess with it, just use it.

Turpentine will clean the brush. You can get a small bottle of it with model making paint sets.

Side note: It is mostly useful in preventing corrosion, which will destroy most SMD boards when exposed to water. An alternative is to immediately scrub the board with a toothbrush and alcohol after getting wet.

I just grab all of my significant other’s brushes once they’re past their prime for arting, and use them once and pitch them. But, given that I use 99% isopropyl alcohol to remove conformal coating from a board when I need to resolder something, I would assume that it would also work to clean a brush.