It has been several years. I recall adding a little thinner (what Interlux suggests), but only when needed. I recall starting out full strength on both jobs and adding a little thinner to the tray if the foam brush starts chattering. That happens more on a hot day and if you leave the paint in the tray long, so depends on how fast you are. In sum, you add thinner only as needed to keep the paint at the original consistency. I will probably have to re-learn it for my Dart interior and exterior, but will do the floor first where mistakes aren't seen anyway.