On Sunday, July 10, 2016 at 11:06:27 AM UTC-4, Terry Coombs wrote:
trader_4 wrote:
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?

You're wrong on this one Trader_4 , regular drywall compound WILL dissolve
with water . The fast-setting types won't , they're as you say a
plaster-type set .

--
Snag

Well, I did include this part:

"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."


I know it's not totally hard forever like concrete, but I thought from
experience that when it's dried on tools it's been a pain to get off,
even when scrubbing and using hot water. But it does come off at least
under those conditions with some work, so maybe you're right, with
enough water and enough time, maybe it will go fully plastic again.
But the water exposure the OP had is very minimal and if what's happening
is normal, then you'd have that problem with any latex paint, right?

I just started a test. I knew where there were a couple globs of it left
on a piece of cardboard from a home project here. I took two small
pieces, they are soaking in water. I'll check at 15 mins, see what happens
and continue.