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!