I commonly do this on maple fingerboards, though I use polyester or catalyzed urethane because it's harder. However, the process is still the same. Get your fingerboard all slotted, radiused, and level then shoot the clear. Let it cure well. Once cured sand the fingerboard once again to true it up. Use your fret saw to clean out the slots and finish the depth, and rub it out to your desired sheen. Install the frets carefully so as not to dent the maple and finish on top of it. Denting the finish can cause it to pop up off the wood underneath.
If you don't have some sort of fret press then I can not comment on the hammer method, though the precautions I mentioned would be the same.
I've heard of some companies that laquer/urethane/polyester over verything including the frets. I honestly don't understand why though.
-Doug