I've explained this technique so many times over the years I'll keep this version super short.
In the "old days" this was a standard way of putting a smooth layer of gel coat on wood boats. It was called "cello finishing" because a layer of plastic was used to cover the gel coat. This gave a very smooth finish when the cellophane was peeled off.
I took my ocean going boat across Europe from the Med. to the English Channel through hundreds, yes hundreds of locks never meant for ocean going type boats. When I got the boat back to Canada I had dozens and dozens of nicks, cracks and other nasties to repair.
Short form: Vee the crack if it's really fine. Obtain a small amount of the correct color gel coat. Wash the area with acetone. Very carefully abrade the edges, only about 1/4" to the side of the crack. Get some acetate sheet. Old overhead projector sheets are ideal, if not use stiff poly. Cut a piece about 1/2" larger all round. Put masking tape on 3 edges. Put a drop of gel coat on the crack. Put the poly over it with the top open. Smooth the poly upwards moving any air up and out the open top. With practice you can do a repair that requires almost no further steps. If the repair is "proud" of the original surface then wet and dry sandpaper and buffing will make the repair invisible. The only variation is if the crack is deeper than the gel coat. In that case use bondo to fill to the gel coat level.
And that's the short version
But seriously, no kits etc. are required.