Remove the chrome entirely, then prep it like normal bare metal. No painting over chrome. Costic or Muriatic acid might work, otherwise you'll be sanding for a while. Chrome sucks.
seth is tecnicly right but i had a sherwin williams rep tell me that you can use a red scoth bright pad and scuff as much as you can then use e2g973 vinal etching primer then paint like normal they will not warrenty it but he said he did it to his chevy truck bumper 4 years ago and it is still holding up really good so its up to you
i have scuffed with a red scotch brite and epoxy sealed it before and havent had any problems but to each his on the smart thing to do whould be to strip the chrome.
i painted my chrome taco bumper back when i had it and i DA'd it with 80 grit and primed/sanded it couple times and it never peeled..had some chips from all the miles on it..but never had any problem with the paint sticking to it
seths right best to strip it, but i also painted some chrome on a bike rear fender struts da'd with 80 and used dtm primer im not recomending it but it worked for me been 3 or 4 years
i agree with seth with stripping the chrome. but i painted the chrome grill and bumper on my ram with stripping. i sanded the piss out of them then metal etch primed them. then i primed them with a 2 part primer and sanded and painted. the bumper has been done for about 2 months and the grill has been done for about year. i havent had any problems so far.