I have had little luck in "really" cleaning up a yellowed plastic item. Most
of the good suggestions and warnings have been posted.
If I really want the item to look nice but am not concerned with
authenticity, I have found that the new "molecular bonding" paints do a nice
job (once you have practiced with applying them to textured material).
[Off the record...And I WILL deny this...My Mother-In-Law is really into the
"arts and crafts", her painting skills far exceed mine, so I "let" her
do
this, and the results are almost always quite good!]