Shellac or lacquer are the older tried and true methods. Their dielectric constant is very low, their dielectric strength is fair.
I use Krylon Anti-UV Gloss coating, just a Light Shot to hold it in place. (ACE hardware). It Stays Clear, in a thin layer it doesn't even show. Prevents tarnish, I use it on all copper and brass except where a connection must be made. Its heat resistance is good.
Varathane has a high dielectric constant and will cause a bit less efficiency. I yellows, too.
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