I noticed that racing sparks have burned themself into the surface of the varnish on the secondary coil of my first DRSSTC. This Tesla coil have seen the most use, both indoor and outdoor, have experienced lots of racing sparks and despite the abuse, runs very well. On a cold day, I breathed on the secondary coil to check on a mark in the varnish and I was surprised by the amount of burned in sparks that appeared!
Tesla coil secondary with marks: https://kaizerpowerelectronics.dk/tesla-coils/kaizer-drsstc-i/
Tesla Coil racing sparks
One my secondaries with a lot of use, looks perfectly fine. It has a thick layer of multiply coats of polyurethane, brushed on. It’s great fun to breath on it, when its a bit cold and you suddenly reveal thousands of spark traces burnt into the surface.

And another image of sparks burned into the varnish

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