Fabulous response! One vote each for paul @2, mattrom @27, peterm @12, fiery @28, spike @22, and rossim @23; two each for ginge @16 and mattrom @10; a whopping ten all told for mattrom (with one unspecified from ginge), including six for the winner:
Painting technique involving many layers? (3,7)
For clarity of tone it’s unmatched. I work in oils but am in awe of this medium.
https://www.youtube.com/watch?v=yyWK14rNGd8
Thanks to all for your brilliant clues- most gratifying.