Your description of the last coat sounds like my experience painting my good old '51 Chevy pickup a few years ago. Before the last coat I sanded it to take off the glaze. When the paint went on it looked good for about 20 minutes then went rough. Lots of swearing and sanding then the last application and it did it again. It made no sense so I took a small panel and did some tests. Contrary to the instructions with the paint I was in a gray zone between too soon and too late. The paint was not fully cured so the thinner ate into the earlier coats. At the micro scratches the thinner penetrated more deeply which caused the roughness. I let it set another day before the final sand and application and everything went well. Don't know if this sounds like what you had but thought I'd throw it out there. (P.S. See Westie's comment )