The soldermask paint i got is super thick, nearly thixotropic, and the limited listings said not to use a thinner.
I've also bough a cup of "blue ink" that was described as mainly a photoresist and was intented to be used with a thinner, as well as a bottle of "blue ink thinner", I suspect these are all the same type of stuff but we'll see when the blue stuff arrives.
The tape as a spacer was a good idea, but I buy super cheap tiny 50x70mm or 70x100mm scrap boards so I'd be doing a lot of taping, and I think even then the layer would be too thick.
So far using a stainless steel pin/tube as a roller gets me a thin and sort of uniform layer, but with a texture - then applying the plastic film removed before etching sort of smooths it out, but the end result is still quite thick.
I'm hoping the paint isn't too thick to spin coat (i guess its mainly a matter of RPM). Stubmled across an instructable suggesting to use old hard drives and drill out the centre of the motor shaft to create a vacuum chuck which sounded brilliant. Until I realised all my "old" hard drives have fluid bearing and drilling through the seal breaks the bearing.
Have a DC brush less motor and a stainless tube that is the same diameter as the shaft - plan is try to put outside or inside threads on the tube, and replace the motor shaft with the tube. Put a shaft seal on one end and a surface to form a vacuum chuck on the other, and cross my fingers. If that works I might try to use a hard drive motor with a pulley just to get something low profile and roughly the right speed range.
Or maybe see if I can find a place to go dumpster diving and recover a pre 1990's drive without fluid bearings.