"Hot coating" as I know it comes out of surf board shaping. There are some good videos on Youtube. After glassing they will apply a fill coat by pouring on resin and spreading it with a squeegee. After this has tacked up they will apply a "hot" coat over top by pouring on more resin and distributing it with a brush. They then tip off the surface and let it cure.

I have tried a similar technique on kayaks but due to the more vertical sides it wasn't great. Surf boards are mostly flat and the resin can level out nicely producing a smooth, glossy coat.

My understanding is most surf boards do not get any further finish beyond the hot coat, i.e. no varnish. Some shapers may rub out and buff the resin for the best looking finish. The same techniques are used for epoxy or polyester resins.

Since foam surf boards don't have that long a lifespan, the lack of UV protective finish is not considered a big deal.

I have not heard the term "hot coat" used in relation to varnishing, but you need to be careful about re-coating varnish before it has fully dried as it is possible to trap the solvents in a deep layer by sealing it with a fully cured top coat. The result can be a weaker layer of varnish below the surface which may cause a finish failure later on.