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Catalyzed polyester resin based products (Gelcoat is polyester based) cure in the ABSENCE of air, so some medium is needed to block the air from getting to it as it is curing. In a mold there is no air between the mold and the part so it cures. When you splash a part off of an existing part, there is no air between the existing part and the splashed mold you are making so it cures.
And Yes, PVA and Mold Release wax is ALWAYS needed on the surface you are going to spray onto. Don't skimp here or you'll be sorry. The more layers there are the better.

Sometimes when in need of a perfectly flat piece to tab into a hole I shoot directly onto a flat sheet of glass to get a perfect surface. Then I add the fiberglass / resin schedule on top of it the tacky gelcoat. Again on the gelcoat side of the part there is no air so all is well... But on the last polyester resin/fiberglass layer, there is air so an air barrier is needed for a good cure.

In all cases you have to prevent the air from getting to the polyester resin base. WAX IS USED TO DO THIS. When a small amount of wax is added to the catalyzed resin base the wax is squeezed to the surface and forms the air barrier you need. You must wash it off between layers in order for new layers to bond the prior layer. Napthaa will do the job. OR better yet use UNWAXED resin (gelcoat) and just pray it on when the prior layer is still tacky. This will give you a good chemical bond. Then the last layer will need the WAX added for it to cure properly.

NOTE: Surfacing Wax is NOT mold release wax. It commonly comes in little brown glass bottles or is already in the gelcoat when you buy it from your supplier. I buy UNWAXED Gelcoat along with enough "Surfacing Wax" to handle the amount of Gelcoat I just bought.

NOTE2: Polyester based resins are catalyzed by MEK-P. Again, gelcoat is a polyester based resin with bits of exotica added to make it look so nice. MEK-P is time, temperature, and light sensitive. So store it in a cool, dry, dark place when not using it. A refrigerator is perfect. Even with storage in a refrigerator it only lasts about a year. Much less if you store it in your hot garage in the sunlight.

Improper MIxing is the leading problem with gelcoat failure. Stir catalyzed resin without bringing up bubbles but stir it till it is mixed thoroughly.
And be sure to use a timer to count down how long you have before it kicks. Once it kicks, your done, If it kicked in the gun, some of the expensive guns may present you with a real problem. You can delay the kick by working cool, taking care with the catalyst to resin ratio, and keeping excess catalyzed resin/gelcoat in the widest bottom container you can find AND keep that container in ice, and switch out & clean guns early and often. Every Little bit helps.
Practice, Practice, Practice and it's a piece of cake.

Rit