Many years ago we regularly used platinum crucibles in many analytical chemistry procedures, and according to my analytical chemistry book, platinum can be cleaned by fusion with potassium or sodium bisulfite. I later adopted this procedure with good success to clean the platinum apertures for an RCA EML electron microscope.

Get asmall ceramic crucible, put a bit of the bisulfite in it, heat it until the bisulfite melts, then drop the apertures into the melt. After a few minutes allow the melt to solidify, dissolve the bisulfite with hot water, and your apertures should be as bright and shiny as new.