Gelcoat is available in 2 formulas, one has wax (finishing), one does not (layering). Both need to be mixed with an agent to cause it to harden. Gelcoat that is exposed to the air stays tacky, this allows for a chemical bond to form between layers. When using gelcoat for the finishing coat of a boat repair, the common method is to use the formula with wax. As the gelcoat sets, the wax floats to the surface and forms the barrier that allows it to fully harden.
While shopping for a product at a popular web seller, I found this review for a verified purchase of TotalBoat White Gelcoat.
Was happy at the time of application on how it was ..., October 20, 2016
I had called the company to make sure I understood how to apply product and mix contents. Got the boat ready and applied the gelcoat. Was happy at the time of application on how it was going to work, but when came back next day there were spots that had not hardened and still have not. I do not understand what went wrong and will be calling the company back on what possibly went wrong. I mixed hardener the same, the surface was all prepared the same. Will have to redo the boat and not going back with this product.Source.
According to the information on the product page, this gelcoat was made with wax. I searched the site where I found the review, but couldn't locate a wax-free product. I ended up going to another source to get a wax-free product for my first layer.
Obviously I don't know for sure, but the description of "spots that had not hardened" makes me wonder if maybe they used soap and water (acetone is recommended by some) to clean the boat prior to applying the gelcoat. If so, and some soap residue remained, it may have potentially risen through the gelcoat, repelling the wax in "spots".
Assuming the person mixed the gelcoat and hardener well, what could have caused some areas not to harden?