Proper machine tool for sanding metal frame
You do not need to strip all the existing paint off, though you can fairly easily with a chemical paint stripper if you want to. 

You need only to remove the shine from the existing paint; what I call de-glossing, to prepare it for a new paint coat. You should be able to use the 180 paper to accomplish this, but utilizing successively finer grit paper will incrementally improve the look (smoothness, shine) of the new paint.