If it is sanding fine, then just scuff it, wipe it down and spray on a bunch of 2k. Then start block sanding. BTW, only use the DA to knock down filler. If you use it on the 2k you will have a panel that has waves in it. Only use the long dura block.
You shouldn't have any problem sanding the 2k, I would use 320 and it would smooth out nicely. I don't understand why you are having a hard time getting it smooth. 150 sounds incredibly aggressive unless your going down to metal.
Good luck.
