How thick is this epoxy?
If it is very fluid, it might be an advantage to pour it on the fretboard, as it would make a smooth surface and reduce the amount of sanding you have to do.
If its thick like the epoxy i've been working with recently:
Brush on - Let dry - sand until smooth - and repeat the process until your desired finish is acheived. To get a glossy look with this method, you would have to buff up the epoxy afterwards.
BTW, you dont have to use a brush. A putty knife, or even a sheet of relativetly thick paper would work.