I have used all sorts of jars with both metal and plastic lids, for jams and chutneys.  However, I noticed a couple of weeks ago that one of my few remaining courgette chutney jars from last autumn had a layer of mould, and that was in a peanut butter jar.  It's the only one that seems to have gone that way, though and, when tested, the lid didn't seem to be a very tight fit...when I thought it was screwed up to its limit, it kind of carried on past there, if you see what I mean.  So that's probably why it happened.

I'm with Kate on this, I use the cello discs (dipped in brandy) then put a metal or plastic lid on top rather than an elastic band. This generally works pretty well. One of my friends uses a lot of honey that comes in jars with plastic lids and it seems to work fine when I re-use these.

Yes I do that too but in the past I used to melt parafin wax over a disk of tissue paper dipped on brandy. I liked this method because the chutney never dried out but it grossed my friends out.......