rivendell has always suggest some sort of contact cement or glue to get the grips on there. the idea that i'd pay the premium price for cork only to have them be useless once off never sat well with me, so i've found that a mix of various tapes (cotton tape, tubeless rim tape) are perfect for making up the slight variance in OD / ID between the two.
as for shellac, i vote clear. amber works if you're trying to change the color of something (like bar tape) but clear just adds a nice and classy sheen. the cork is already brown...any browner to me comes off as old fashioned instead of classy.