Go to your local paint supplier and let them know what you are doing. They will set you up with everything you need to get the job done properly. Sam is right, flex additive is for painting flexible parts when they are off the car so that the paint doesn't cure hard for about a week. This allows you to get the bumper into position without having the paint crack and delaminate from the surface. Use it if you want. It is much more important to get the right primer on a plastic bumper so make sure it is a urethane cover and not something else. Also scuff the hell out of it for a good mechanical adhesion and wash with soapy water in addition to a wax and grease remover. Good luck
(one thing about wax and grease remover... on plastics where there is chance for static electric sparks you need to use a cleaner that is safe. Not flamible. Every company makes a cleaner just for cleaning plastics. I use them and would recommend them for saftey sake.)
William McCormick "I am not an expert, but I do know some basics that are often not known by many."