FO is cheap. Silverprint is finally selling the stuff in the UK now (God bless 'em). My advice is to buy it in powder and, as Trevor suggests, make up a few months supply and replace it every 4-5 months. You could get away with longer, but in my experience, it's not worth the bother. Getting it into solution is no where near as painful as it's reputation predicts; in fact I have more problems getting the pd salts into solution (always end up filtering out metalic particles - still don't know why ??). A dash of EDTA helps, but that's a matter of personal choice.