So, I am a long time Shortcake (R/w painter) player who has just recently made the switch to a U/R/b based deck. There were a couple of other people wanting to see the list and I thought it makes more sense to post it on this forum than the imperial painter forum as the base is pretty much the same. So, this is where I am at with my list right now:

// Lands
2 Great Furnace
3 Ancient Tomb
1 Swamp
1 Mountain
1 Island
2 Volcanic Island
1 Bloodstained Mire
1 Underground Sea
2 Seat of the Synod
1 Badlands
4 Polluted Delta
2 Scalding Tarn

// Creatures
4 Painter's Servant
4 Goblin Welder
2 Baleful Strix
1 Trinket Mage

// Spells
4 Grindstone
3 Pyroblast
2 Ensnaring Bridge
2 Mox Opal
2 Lion's Eye Diamond
2 Tezzeret, Agent of Bolas
2 [CN2] Daretti, Ingenious Iconoclast
4 Force of Will
1 Engineered Explosives
2 Intuition
4 Brainstorm

// Sideboard
SB: 3 Blood Moon
SB: 2 Surgical Extraction
SB: 1 Pithing Needle
SB: 2 Flusterstorm
SB: 1 Trinisphere
SB: 1 Toxic Deluge
SB: 2 Kolaghan's Command
SB: 1 Sudden Demise
SB: 1 Notion Thief
SB: 1 Nihil Spellbomb

I think I am 14-2 with the deck so far. I had a loss to burn which I think is just a bad MU (haven't found a painter deck that likes burn) and a loss to Shardless BUG where I just got tempo'd out and didn't see a blood moon. I have tried straight u/r painter in the past and, although the speed to combo is much better than shortcake lists, it just felt really fragile. The planeswalkers in this deck give a very good alt win condition and make the deck much more resilient. I can't stress how good Daretti in particular is in this deck. I would like to see a third ensnaring bridge somewhere in the 75 along with a fourth grave hate piece but not sure where to put it. I haven't gotten to actually play with notion thief yet and it may just be cute. That would probably be the first card to go in the SB. For those who expressed interest, please give it some play time and let me know what you think. I am always open to suggestions.