Painting - Refinishing large, round bannister head

A neophyte painting question.....One of our large, round, wood banister heads is showing wear and tear and I would like to refinish it. The white semi-gloss enamel paint (at least I think it's semi-gloss) is wearing thin in spots.
Should I simply take some sandpaper to it, prime it, and slap another coat of semi-gloss on it? I'm concerned about it matching the rest of the bannister. Also, how can one distinguish between semi-gloss and gloss enamel if one isn't sure what was used originally? Thanks for any help.....

You got the right idea... sand, prime and "slap" a coat of paint on it. :D It will probably be impossible to match the old finish. Pickup a few sampe charts at a paint store. One manufacturers semigloss is another's gloss. I think as long as you leave a neat separation point between the old and new paint ( at the base of the banister head), and as long as the paint color is fairly close, you'll be ok