Hi Mick,
I do not prep my Diacov before painting, and no problem, but it is apparently recommended for you to do by the manufacturer. So up to you
No need for Balsa Lock etc. This is full size stuff, that thing stick like nothing else
Now, that depend if you use Diacov or Diatex. Diacov have the glue backing, Diatex doesn't and you need to brush the glue on the surface