On Sunday, July 10, 2016 at 10:20:08 AM UTC-4, dadiOH wrote:
"Percival P. Cassidy" wrote in message
...
I repaired some holes in a wall that is to be wallpapered, and then started
applying "Shieldz" to provide the proper surface for the wallpaper -- and
now, an hour or so later, I find that the drywall compound is lifting up
and falling off at the nicely tapered edges. The dry-wall compound has had
a couple of days to dry.

What have I done wrong?

My guess is that the water base Shieldz transmitted enough water to the thin
areas of compound to turn it back into its original state. If that is so, I
wouldn't expect it to fall off though, just stay where it is and dry again.

A coat of shellac before Shieldz would solve the problem.

Drywall compound is not supposed to turn back into it's original state
with the application of some water, certainly not with the amount of
water in some primer that dries in 15 minutes. AFAIK, it's like concrete,
ie it undergoes a reaction and it's not simply water going out and it's
ready to accept water and return back to original. The problem seems
to be with whatever the joint compound used was and/or the condition of
the drywall board. We don't know what kind of compound, board, etc.
New or had they been sitting around, surface possibly soiled, etc?