I just tried some of that clear stuff and it worked great. Takes forever to dry and would probably work better if baked.

I've always been under the impression that hearing buzzing from an H39 or any MV Ballast is fairly normal with age.  Growing up, there were some that I could hear as they were warming up.  Never heard them from the HPS Ballasts.  Then again, some of those GE Post-Top lights had been around since 1963 in my neighborhood.  Come 30 years later, they were buzzing pretty loud, and once I started hearing that buzzing and seeing the blue-ish glow, I knew it was time to get home. 

Some of the older fixtures tent to have water seepage and therefore the ballasts will rust. That coupled with upwards of forty or fifty years of nightly use means that the ballast varnish will be pretty dried and the core is rusty. Sometimes a fixture in good enough shape after a car collision or vandalism may be reused instead of replaced. The jolt from the car hitting the pole as well as the lantern falling to the ground could knock the windings loose or dent up the core. Rick delair said that he had a fixture that was thrown around and smacked by vandals and buzzed loudly and was a little toasty. A varnis application and being used following the application rendered it "quiet as a church mouse".