Thank you Gents,
One thing that I don't think really shows is just how thick the blue paint is on the white, it really is noticeable to the touch, maybe 0.5mm, and that is after I've spun it at high speed.
Chalk paint appears to be Emulsion (what the Americans call Latex) paint with a food grade calcium powder added, it really is thick and difficult to apply smoothly. The bowl itself was painted a couple of times and sanded back carefully to get a smooth but not back to wood finish.
I think I'll persist with this idea for a few more bowls, I like the effect it gives but next time I'll thin the paint and see if it moves better, trouble is I might get more bleed on the thin lines