OK, I have to admit my "dirty little secret".  I don't consider it important - at all - to
use any kind of "prep" for decals.  I float my decals on using just distilled water.
Then I wick off the extra.  Then I apply Solvaset and wick it off (using small pieces
of tissue held in tweezers).  Let it dry (mostly).  Reapply more Solvaset and wick
off the extra.  Let it dry (again - mostly).  Use a pin to prick the surface of the
decal (round headed sewing pin).  Reapply Solvaset and wick.  Let it dry.
And "done".  Let it dry - FULLY.  Use pure - distilled - water to clean up any
residue from the Solvaset.  Really DONE.  Yes, I use the same process for
all decal brands.
  Your methods may vary.  Mine work for me.  
                                                                                                       - Jim