You have to overcome the bond created in the stopper threads from the solidified Watco finish. If hand twisting will not do, perhaps a pair of pliers will do the job. Better not have this occur in the first place by applying grease to the threads after each use and before putting the cap back on. This, however, has not been a long term solution. I have now resorted to using small empty paint cans to store my decanted finishes, thinking that the pry off lids will work better than removing a screw off cap. Will still fill up to the rim with water to minimize the amount of oxygen above the finish and under the lid. Jury is still out on this method.