Hello evey one

I have done a few lost foam casting, witch turn out good, I am using the white polystyrene foam, and I am happy with the results.
My question is
Is it normal to have small little air bubbles in the casting when using lost foam ???

I also degas my Aluminum right before pouring, and I pour at about 750oC to 800oC

I also tried to coat my foam with plaster, and take out the foam with acetone whats left is some white slime around my plaster mold, will that cause a problem ??

Thanks in advance
Biggs