Craig, to do it properly you need to part the joint and apply the paste on the inside of the pipe. I can't see applying paste from the outside doing anything. Of course you could wrap it and use a clip as you suggest but it's a bodge and will look like one. Sometimes the joint is 'sprung' and once you loosen the u clamp it opens up a bit so you might need to push the two together as you tighten it. If that makes sense. I've also found that the pipes can be too loose a fit especially if of different origins.. I've cut a thin strip from a coke can or similar to use within the joint as a fix.
