Am decorating a replastered room. Since the ceiling meets the external
wall on a curve rather than the usual 90-deg corner, I had to decide
arbitrarily where to delineate the wall and ceiling for decorating
purposes (which I'm doing using different-coloured Leyland-brand matt
vinyl emulsions.)

So - I painted the whole lot with a sealing ****-coat (the emulsion
thinned down by 20-30%), then 3-4 days ago gave the ceiling one coat of
brilliant white. This morning I painted the walls in apricot white (OK,
that's magnolia to you and me!), having run a line of 1" masking tape
(3M brand) across the top of the wall to make a nice sharp edge to the
wall colour. So far so good.

This afternoon everything was at least touch dry (it's been damned hot
here all day) so I peeled off the masking tape. Beautiful sharp edge
did indeed result - but unfortunately the tape removed nearly all the
underlying brilliant white, leaving bare plaster. ********, ********
and ********.

So WTF went wrong? Was the plaster not properly sealed - did I use the
wrong stuff? Or was it not properly 'set' even after a few days; ie
should I have waited longer before applying masking tape? Would be
interested to know for next time.

Thanks
David