There’s now one less thing to worry about when painting, and it’s how much water you can or should mix with acrylic paint without ruining its adhesion. Golden Artist Colors (a USA employee-owned company renowned for its artist’s quality paint and techical info) have updated their advice:
“For years our standard advice was that a 1:1 ratio was very safe for most of our paints and mediums; plus, it had the advantage of being easy to remember while greatly erring on the side of caution. However, our current testing shows you can go a lot further than that before encountering significant issues. Just how far? We think you will be surprised.”
The article gets into the specifics, but for me this is the takeaway:
“We got no adhesion failure of any of our paints, no matter how thinned down with water, when applied on top of acrylic gesso.”
In the FAQ on thinning acrylics I wrote for Painting.About.com in 2006 (my original version, as here, not the current surreal rewritten-by-who-knows-who version) I’d said this:
“When it comes to thinning acrylics, the only ‘rule’ is to not mix acrylic paint with more than 50 per cent water. Any more than this and it may loose its adhesive qualities and peel off at some stage. You can mix in as much acrylic medium (glazing, texture paste, etc) as you like because it’s got the acrylic resin in it that acts as the ‘glue’ that makes the paint ‘stick’. (Golden describe their mediums as ‘colorless paint’! )”
If painting on a large canvas, I tend to use glazing medium as well as water to thin paint because in addition to adding “glue” it also increases working time (slows drying). Mostly I simply don’t think about it, and merrily spray paint with water to make it drip and run.
Where I have encountered adhesion issues is with water-thinned acrylic ink lifting as I brush over it, despite being touch dry. Leaving it overnight helps, presumably as the paint binder then cures. I sometimes then also apply a layer of glazing medium with a soft brush, leaving this overnight again, before continuing on top. But mostly if I find it’s lifting — you see the colour appearing on the brush — I just keep going and deal with it.