Bob Oswald replies: Driers definitely deteriorate with time. The time is probably unpredictable, but in general I recommend not more than a year of storage. Personal experience with linseed oil has confirmed that, although the product was definitely several years old. A new can worked incredibly well. I had a similar experience on a guild project with varnish, again several years old. It never did dry and had to be stripped and re-applied with fresh.