This is not a hard and fast rule, it is just a rule of thumb so that you can begin to see the kind of surface preparation you need prior to applying dry micro. Before applying the dry micro, paint the area with pure epoxy. Wipe as much of this epoxy off as you can with a clean paper towel. A good idea here is to only try to do a small area at a time, say a square foot or two. Mix up a batch of dry micro - the consistency of cake icing works well.
Some people try to mix it so dry that it is almost impossible to apply.
I don't agree with that. The gram or two of weight you might save per 8 ounce cup is simply not worth the enormous effort. Squeegee through the peel ply to get the micro even and where you want it. Once the whole surface is micro'd, allow it to cure. Contour sanding should be done using a long sanding block.