Surfboard resin does indeed harden through a chemical reaction. Usually the process involves mixing together two chemicals, one is made up of long molecules calledpolymersand the other is a cross linker made up of molecules which bond easily to the polymers. When you mix the two chemicals, the polymers tangle and the cross linker secures the polymer in these tangles. The end result is a material that is very hard because a lot of force is required to break all the tangles.

Putting the resin in the sun can speed up the reaction because the extra heat from the sun makes it easier for the cross linker to form bonds with the polymers. Also, there are UV catalysts which use ultraviolet light (the same light that gives you a tan) to create molecules which make it easier for the cross linker to form connections. Thanks for your question!