XF-21 does have some white "Stuff" in it, I looked at mine and it's the same way. When you said white paste I imagined something like toothpaste, sorry for the confusion.
Windex will strip the Future off, but it may take the acrylic paint as well. Alcohol will probably work as well, but I'm also afraid it will wipe out the acrylic paint.
Buff, I apologize for pointing you in a direction that caused problems. I've used Future and XF-21 a bunch of times and never had problems. If I had thought that it was going to mess up your model I never would have given you that tip.