15 years old???
Polyester and epoxy resin systems both have a shelf life, but neither of them are anywhere near 15 years.
Polyester has a max shelf life of 6 to 12 months from date of manufacture and the MEKP hardener 12 to 24 months. Sometimes you can use up old resin by using increased amounts of fresh MEKP, but even if you can get the resin to set it will be understrength, so don't use it for anything important. Polyester resins are the ones with a 100:1 or thereabouts resin:catalyst ratio.
Epoxy resins have a much better shelf life than polyesters but 5 years would be pushing it. Epoxies are typically the systems with mixing ratios of from 1:1 to 5:1
Both systems basically just lose their reactivity over time.
If you are talking about a kit I guess we are talking fairly small quantities, so its best as others have said to buy fresh materials and start again. Scrape off what you have done and clean up with acetone for a polyester system and MEK (that's MEK not MEKP) for an epoxy system.
Take some clean empty glass jars to your local fibreglasser and get him or her to fill them for you, he or she will be using fresh materials.
Avoid the prepacked kits from the big box stores, they could have several months on them before you buy them and may have been made with old resin anyway. The big box stores put severe price pressure on the guys that make up the kits just like they do to every other supplier.
To maintain margins, the kit makers will buy the cheapest resin they can, which is quite often the out of date drum that has been forgotten in the corner at the resin factory. Or maybe it is the drum returned by the fibreglasser because there were problems with the resin. As long as the resin is still liquid and can be poured into their little kit tins, what the hey!