I follow Dave's method. The polyurethane (I have also used spray shellac) gives the copper something to "seat" in, as so that even if it does expand a minute amount the diameter of the coils does not become greater than the diameter of the form. I usually place a couple (2) of light coats down first. I believe this also helps with insulation, as air is not trapped between the coils and the form so much.
Loren
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