this paint sounds very much like createx to me,, acrylic polymer.that dries matt and is soft untill fully cured

if thats the case forget about thinning with water ,windex,..or grandmas recipe.Are we still in the steam age ?

get yourself a bottle of  "acrylic airbrush medium" to thin with, you can find several brands at hobby lobby,michaels..ect....it's easy to find and try

future works because it's acrylic

thinning with water/windex/washer fluid/whatever/ will work to a degree but it also weakens the paint by stripping it of binder

(what allows the acrylic to cross-link , cure, and  dry properly).these are not the acrylics of 15 years ago

 acrylic airbrush medium solves that problem.it's basically acrylic paint without the pigment.(kinda like future)

if it works with createx ,,I'm betting it should work with these ..if not it's only a bottle of medium wasted