[Matty4]

Yep. If they're dull and yellowed, you can sand that off and then polish them back to clear.
Depending on how bad they are try using some 1500 wet and dry with plenty of soapy water, progressing to 2000 wet and dry. Then using a buffing machine with some fine cutting compound will have them looking great again. You can also look into re applying a UV coating to try and prevent them yellowing in the future.