Hi there, if your talking about house paint (oil) the only way to thicken the paint if it was thinned too much is get the paint as cold as possible or add the same paint from another can and try to get as much of the pigment from the bottom of the unthinned can into the thinned one mix well back an forth.I've been in the business almost 35 yrs. The only time I run into that problem is when it is over thinned or it is not stirred properly.It will work as long if it's not too thin.