They make a silicone rubber that does NOT excrete Acetic acid during curing.
GE at one time made it under the name "Sil-Pruf", iirc.
Picture of the spot?
The RTV you buy at the local aquarium shop does not excrete acetic acid (aka vinegar). Used to repair and build tanks.
I bought some....and it smells like vinegar !
Back in the day, Dow corning made a spray bomb called... 'HV acrylic plastic'. I sprayed that stuff on anything..and everything. (clear finish). Buddy had a variable cap on his shunt fed 160m tower... that would arc with just 100 watts. I sprayed it with the DC product. No more arcing. Then I dragged over my 1 kw out amp ( that had 160m on it)..and we stuffed 1 kw into it for a week, no arcing.
We even tried it on broadcast air variables on a fellow's FL-2100 B. ( .030 spacing). At one point in it's rotation, the plates would rub...and short out. Ran it fully UN meshed and doused it with the spray bomb. Once dry, runs fine at 650 w out...( with plates still touching..and rubbing).
I believe they don't make it anymore.... I searched everywhere. Corona dope would be a 2nd choice. But the spray bomb was handy.
Jim VE7RF