I've let the outline crust over a little bit on occasions but I don't think you have to.
I don't apply the backing to mine, but I would say you are using it to even everything out and that you want to keep it on the thinner side. The more backing you add the thicker the finished product will be, and while you do want the whole thing atleast a 1/4th of an inch thick it doesn't have to be really really thick.
I use a soft medium for my transfers.
I pop my in the freezer for few minutes between colors or layers. Seems to help.
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