Just because it's hard doesn't mean it's fully cured. As MX stated poly resin is Air Inhibited cured. The Wax floats to the surface and keeps air from getting in and allows it to cure faster. With no wax it will remain tacky and will take longer to fully cure. Getting it wet during the cure time can cause issues.
This should be informative...
About Polyester Resins:
The majority of all fiberglass parts are constructed using
polyester resins, it's the most widely used type of resin in the composites industry. Polyester resins require a catalyst to cure or harden, typically methyl ethyl ketone peroxide (
MEKP). They are naturally UV resistant, generally viewed as easy to use, fast curing, tolerant of temperature and catalyst variations, and they are less expensive than epoxy systems. Many view polyester resins as being easy to use because they have a high thixotropic index, basically they do a good job clinging to vertical surfaces so the resin won't run ordrip as much when making parts and molds. They also wet-out fabrics quickly and easily and they are easy to mix. As the most common resin type in the industry, it can be difficult to narrow down a list of common uses. That said, you can expect to find polyester resins used in the marine industry, used for boat hulls and other applications, they are used for general part fabrication, automotive applications, and for low-cost molds.
While these positives obviously weigh in polyester's favor in many applications since it is so widely used, there are a few drawbacks which must also be considered. For one, polyester does not offer as much corrosion resistance as vinyl ester resins, or as much ultimate strength as epoxy resins. Also, thin applications of polyester resin can remain tacky when exposed to the air. We recommend adding
Styrene Wax to your resin for thinner layers. The wax rises to the surface and provides a barrier to the air which allows the resin to fully cure. Note, the wax must be sanded and washed off before application of the next coat of resin or paint. If a laminate or mold is constructed using three or more layers of fiberglass reinforcement, no wax is necessary as the heat generated during the cure cycle will fully set the resin.
Tech Tip:
If you want to avoid adding wax to your thin polyester layup or repair, spray
PVA mold release over the lamination or repair and the resin will cure tack-free. Wait 25 minutes after finishing the lay-up, then spray on two light coats. Within a few hours, the PVA can be washed off with water, leaving a fully cured resin or gel coat surface.
