I can only presume the HF ran through the can and blew a hole (and
sparked) as it jumped to the sculpture.

I can see that happening on my welding table. Over the years , (being the
untidy slob that I am) an accumulation of electric conducting debris has
collected on my welding table. Several times Ive lifted my welding hood to see
3' lengths of filler rod glowing red. This has inspired me to keep my propane
torch off the table. I can see whereas you probably have your welding ground
clamped to the table, the paintcan just became part of the circuit.