Hi, your first batch was too old. The shelf life for polyester resins is
maybe 6 months although it may harden to the point of uselessness before
that. And your retailer may have given you pretty old stuff. But it sounds
like the second batch was fine. There are two types of gel coat; brushing
gel coat and spray gel coat. What you bought is probably brush gel coat.
Spray gel coat is much thinner. Its essentially the same formulation but
they use silica and other additives to thicken it. Usually, I just buy the
one I need. Adding acetone will not work; it flashes off (evaporates) too
soon. You can try adding neat resin but it may not be worth while; just buy
the right stuff and use it with confidence.
Gel coat repairs are not that hard and one can usually achieve a 100% repair
ie invisible! But you do need to match the color first. The concept is the
same as Bondo repairs.

Arnold