I am refinishing an oak table for a friend. It has been stained and given a few coats of Watco oil. I let the last coat of oil dry about 7-8 days before starting the varnish. Each coat has left "dimples", areas where I have observed the smooth varnish pull away and form a dimple. There are no dimples where the grain is nice and tight, but only where the grain seems to open up. I just put on the fourth coat, hoping they would fill in, but I watched the smooth surface give way as the varnish actually pulled away and left a depression. This is only the second time I have used varnish, the first was on a cherry table and I had no such problems. Any suggestions? Any fixes? Prevention for the future? Thanks in advance.