Retr0bright works well but it was designed for ABS plastic. Not sure if playfield posts are ABS. The ABS yellows upon exposure to fluorescent light which causes Bromine added as a fire retardant, to break free of its chemical bond. This is the yellowing you see. I've used the process quite a bit with very good results. First thing would be to find out what kind of plastic the posts are or just try it out and see what happens.