I'll add if possible (and if offered in the paint line you're using) use flex additve for the covers. One thing to note is it's most likely the paint is laquer so that adds to other things to keep in mind for what can work over it as not all paints like each other. As for those cracks, it is from the laquer cracking & exposing the fine line of the bumper's urethane to the elements. Those can be covered/filled with thin coats of plastic bumper repair from 3M, Lord Fusor & Upol. Like as Rktpwrd & 57 Handyman both brought up, fine sand paper & not getting heavy on the sanding, just something to allow the paint to get a bite after cleaning things up. In an opposite from some of the advice & this comes from old school body guys I used to deal with, sometimes it is not needed to remove all the old paint but think about how the new paint type will react to the old/unkown type of paint. One other thing is try to set up a jig to hold the covers in the right shape to ensure the repairs are smooth & not wavy from them always flexing while working on them.