Sorry, I wasn't clear.
The blush or milky material is a waxy material that comes out on the surface of the epoxy - sometimes.
Your epoxy/ glass is perfectly strong (unless there was some milkyness when you were doing fill coats).
The waxy blush sits on top of the cured epoxy. You can't see it, but sanding will ruin a piece of sandpaper quickly, you will see it on the paper clogging it up. I have also sometimes scraped the surface of the epoxy and peeled up the blush - it becomes very apparent on the blade and will feel a lot like wax to the touch.
When you put another coat of epoxy or varnish on then it is visable. The strength of that last coat's adhesion to the previous layer is very poor.
At that point you have to sand off the last layer of varnish or epoxy in order to remove it, since the wax is encapsulated.

Washing each cured layer of epoxy with water will remove any waxy blush. You don't really need a cleaner but I sometimes use a soppy ammonia. You absolutely do not need paint thinner or other strong cleaners. The Wax is water soluble.

FYI, I once let the initial epoxy/ cloth laminate harden with out filling the weave with epoxy. And I did not wash the surface. I saw the blush when I added the first fill coat. I tried to sand away the fill coat, but of course I could not get all the fill coat off because it was trapped in between the peaks of the cloth. So I had a very nice regular pattern of nasty yellow spots in the final finished coating. Eventually I sanded off the complete glass coating and started again. Silly me.