I agree with all the answers you've gotten .
Just like to add this though . If you are really desperate to thicken paint , Let it settle very well until you see the semi - clear liquid in the bottle .This is the " Vehicle " I/e the liquid of the paint ,or the base .Now remove some of it with - out stirring up the pigment . Viola ; Thicker paint .
NEVER remove more than half , or you'll have an un-manageable mess . I used to do this and still do for making paints I could texture with stipple brushes ( just like oils only with enamel . )
Point here for all , I have been doing this since Testors original Non - Flat bottled paint .Then I would take Talcum - Powder and put it in the paint till I had a good heavy flat - Like for undercarriages on trucks and cars .You know the old " Undercoating "