I wasn’t sure either so I checked another translation which said “Put this on your wound, to prevent gangrene”
So I think it’s using 塗る (to paint/plaster/smear) and 化膿(festering).
jpdb parsed the grammar for ぬっとけ（塗っとけ）as the 〜とく form which is a contraction of 〜ておく (in advance/to prevent) so I reckon 〜とけ is the command form of that.