This seems to be a problem inherent with this product . I solved it by Opening the bottles and putting a foil seal on the top and cranking the top down real tight . This gave me at least a year per bottle . I used a lot of it for court models .
I have even had this happen to a quart sized can of M.E.K. . I impagine a micro tight seal is unobtainable with anything that has this product as a base . T.B.