I used a heat gun. The varnish is in fact plastic. It took me one hour, one hour and a half per drum. And then had to pick up all the (mischievous) plastic chips.
Then I sanded the drums (grits 120, 180, 280 and 500) with an orbital sander and put 10 coats shellack on each drum (you can see the tumbadora on the right).
And I'm now wrestling with a sturdy steer skin. I'm not sure I'll win the fight...

Edit : the steer was a tough one !

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It seems the hosting site doesn't accept my vertical pics...