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duct tape is made out of plastic, gaffer tape is made out of cloth.
duct tape is waterproof so i imagine that's why it's recommended. the roll of duct tape i bought does have bits of cloth running through it but it's a plastic tape, and VERY sticky. i think it works like a waterproof plaster and makes the skin/verruca underneath very soft. they say to keep it on for 6 days, then soak the verruca for 10-15 minutes and try to remove some of it, leave it overnight then put duct tape on for another 6 days (if the tape falls off during the 6 days, just put some more on).
the other advantage of duct tape is that it means the verruca won't get spread around if you walk around with bare feet. it's very sticky tape so i think it'll stay on better than a waterproof plaster.
they reckon even if you do nothing the verruca should go in a couple of months.