I sanded down my stock valences about half way to clean them up. I then whiped them down with 50/50 iso diluted into water for about 15 minutes. After you've done this, you can use high build primer to smooth them the rest of the way in just a few coats + sanding they'll be ready for paint.
If you do paint them at this point with rattlecans, probably should just plan on a lot of wetsanding and buffing because they won't shine like the original paint otherwise.
A lot of people just paint/clear them as is. Of course this looks funny on certain colors like silver for example. Most of the time it looks pretty good though.
Bora Azul in the MK4 forums just painted his side skirts with rattle can bed liner/underbody coating. It looks pretty good but he did a very good job with prep and his work. Still used rattle cans though!
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