Bear in mind that once thinned, you must use it or bin it, it will not keep well. Depending on the quality of the acrylic, it can be thinned as much or as little as you like. However, like all paints, when exposed to light/heat, it can craze and crack without a protective flexible coat - this is the same in Decorating finishes (my trade) or in illustration painting (next, now I'm retired)!