I've used the paste on some of my bottle calves. Sometimes it works really well on one group and all the horns are gone, sometimes I end up with a calf (or two) that still has horns, or just has one short one.
With the four 6 wk old calves I have now, one is completely dehorned for sure, some of the others may or may not be...I either need to redo the job or just wait and chop 'em off later. If I suspect a calf still has horns, I do like to wait a little while to make sure, because a time or two I've been ready to put more paste on, touched the horn buds, and they flaked off. Depending on the paste it will sometimes take a few weeks to know if the horns are gone or not. According to the directions, you should only have to treat them once, but it doesn't always work that way.
I like the idea of using duct tape; haven't tried that before.