Bob,
depending on how large/big the dent is you can also try to use an air compressor to get the ding out. Use a bicycle tire tube or something than can take some pressure.
Put the inflatable tube between 2 rigid parts (best to follow the mast's inner curvature - shape some wooden blocks or something) and get those inside the mast in the ding area - then slowly inflate the tube and check if there's progress.

Bob, I never had to deal with dented masts, this ideea just happened to pop up, it may be very well a bad ideea.

regards,