I'm certainly not a finishing expert, so take this as MHO. I have used ARS satin and Seal-A-Cell on several projects. It is so thin out of the can that I never considered thinning it. It requires frequent stirring to keep the flatteners in suspension. IMHO, the MS has thinned it to the point that something is going sideways as it drys. Just as an FYI, it usually takes 3 coats and sometimes 4 to get a satisfactory finish. I don't know if you can fix it at this point without stripping the ARS. Oh, and I rag it on.