Okay. I very rarely use flap discs and never really understood what all the flap was about.....did you see what I did there?
When trying to finish something flat or near flat I will always use a sanding disc like the orange blaze ones from Norton, usually 80 grit and then finish as required with a DA equipped with 80 grit as well. If I am trying fix something ugly that requires some more aggressive metal removal I will start with 36 grit. 2" and 3" rolocs and 5" discs on an air sander.
The only flap discs that I keep around are 2" roloc style and they are for blending shapes together and nothing else. I have never been able to get anything flat with a flap disc. They can remove copious amounts of metal but the waves it leaves are not acceptable to me.
Something to remember with flappers that I learned the hard way is they are most effective with electric tools. Most air tools do not make enough torque to maintain rpm with any pressure applied. RPM is very important as well. To fast will kill the flap disc and "burnish" metal. I spent a stupid amount of money on a 7" flap disc once and ruined in 5 minutes spinning at 6000 rpm. I bought another one and only ran it on another sander at 5000 rpm and it cut pretty good for the life of the disc, still wavy though.