You could sand them down by hand or with a D/A , the factory uses an epoxy type paint and if you don't get it all off and clean it well it will not let the new paint adhere properly. The best this that I have found is to have them soda blasted , it won't ruin any of the seals and will strip all the old paint off. Then use a self etching primer and a good epoxy primer like DP40 from PPG. then you can use what ever base coat clear coat you'd like or a single stage paint .
Thats just how I prefer to do it.