If you take a reborn out of the oven after it has been varnished and baked, if it has white patches does that mean your varnish/sealer was too thick or that it needs baking again?
Could be either one so I would bake again and see if that works.
Try baking again
I don’t use heat set anything, but have seen this come up enough to know that the answer is bake again.
I bake three or four times.