'Wipe...until set' sounds good to me. I don't think you could say 'to firm the shape' in this case, as you're doing something until it gets firm, not doing something in order to make it firm.
'Firm' can be a transitive verb, but not in this case. (A common, rather informal, use is in connection with arrangements: 'I'll phone nearer the time to firm up the details.')
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