Although it is not quite the solution I was hoping for I didnt realise that the painting method I used was so odd, that's an interesting take-away from this! I shall try to make the canvases bigger! Since the previous post i had done another painting on a larger canvas and using a slighly different method of painting where I use lineart, so the haloing effect is lessened by the edges being hidden by the lineart. But hopefully the larger canvas will mean I can paint using my old method!

Thanks for all your time spent on this by the way, I appreciate it!