We had the exact same experience a couple of months ago with some West Marine
brand resin. Wasted several hours......we bought a can - looked brand new and shiny. Mid-70's temp outside. One drop of hardener in 2 oz of resin almost instantly made a ball in the cup. Like, in 10 seconds, so this is not a "workability" issue. Took it back to West Marine
- they gave us a new can....but they looked at us like "they don't know what they're doing". The new can did the same thing. This time - we had the West Marine employee put 1 drop of hardener - in the ENTRIE can. The ONE DROP of hardener made a white rubber ball in the entire can!!! So there was definitely something wrong with the formula - wonder if West Marine and West System really are the same product and we got the same, bad batch??
But a really good solution - was not to use gel coat at all....we ended up using this - Elmers E765 Fiberglass Epoxy
It was super easy to use - sanded wonderfully. I routed out some cracks and bad spots using a Dremmel - applied the Elmers - two coats ended up giving me a beautiful, smooth surface. I was going to paint
with some left-over white paint we had (two part acrylic
urethane car paint) but the color was off. I ended up using simple Rustoleum satin (so it wasn't too shiny!!) and I can't hardly find the repair and I know where to look! Get the Elmers at Lowe's Home Improvement or similar store.