Plastic nose cones get similar treatment to plastic models:
Wash with old really soft toothbrush usinf either dish detergent or mild abrasive household cleaner.
Scrape/file/sand/fill molding seam marks with Squadron White putty.
(wonder if Hobby Lobby over in city here carries Tamiya putty?)
Sand and rinse.
Prime. Unlike plastic models, nose cones get DupliColor car primer.
for painting I mask off shoulder then tape cone to dowel, tube, stick, something to hold it.
somewhere along the way something got me started painting nose cone bases in colors which look like greenish or yellowish zinc chromate primer. Why? Dunno, just enjoy doing so.