I've had worse. In the RPTV days, some of them would get coolant leaks. The coolant is a mixture of 70 % ethylene glocol and 30 % glycerine. It starts out as nonconductive but when there is a voltage appled it become conductive and corrosive. It also soaks into the board itself.

I used to temperature cycle them a few times to "squeeze" the crap out of the board. Don'tcha think it might be a bad idea to make them our of porous material ? Oh yeah, sure is, wrecks the thing in some cases. In fact if it leaked on deflection or HV it could cause a fire, prompting them to put in a gutter.

Anyway, it doesn't sound like your boards would be impregnated with anything, at least it is all on the surface. I never submerged boards, just doesn't seem like a good idea. But I did take them to the sink and use hot water, and acetone, and alcohol. Of course I had the board clean out of the unit so any components that could be damaged I could avoid easily.

For scrubbing I used a small paint brush with the bristles cut short for stiffness. I started with acetone, then rinsed that off with hot water. Then doused it with 91 % isopropyl alcohol and kinda shook it down. Then a hair dryer. I found I had to repeat the process a few times because of the stuff in the pores but you will probably not have have particular problem.

You can get away with acetone if you don't drown the boards with it.