For AIO I like to use a slightly damp terry applicator. Just works for me better than foam. I dont have any P21S but I would probably use a foam app just like I do with others. QD I like WW towels. Smaller ones.
I've had very good success applying AIO with an brick-type MF applicator that's been slightly dampened. I think I read a post a while back that stated dampening the applicator keeps more of the product on the surface & keeps it from being absorbed into the pad.