Patching and Plastering - Historical Plaster ,need Help Please

please help? historical courthouse cracking apart!

--------------------------------------------------------------------------------

We are drywall finishers and painters and took on this plaster job at the court house. It turns out to be a bigger job then we imagined. One room has 3 four foot holes down to the concrete blocks and hairline cracks the lenght of wall on each wall, some walls have cracks on top and bottom. window sills are plaster and chunked off here and there. we bedded in the giants with gypsum plaster witha little alum sprinkled on to harden on top of the glue brushed on the blocks., we covered with a finish coat of moulding plaster and lime and also used this formula for the deeper cracks, then we just used finish coat to fill surface cracks and feathering. the next day most of the finish coat had flaked , cracked and shattered and is now on the floor. whats left looks like antiqueing or old peeling paint. what the heck did we do wrong aside from taking the job in the first place? And where is the lathing? there seems to be only lathing around corners of windows. please, any wisdom from experience would be greatly appreciated
P.S. OUR FINISH COAT WAS OF IVORY LIME AND MOULDING PLASTER MIXED 70/30
ARCHIE tAYLOR

Was a bonding agent used? I remember from working for my grandpa as a kid that they used some kind of a bonding agent when repairing plaster. Can't think of what the product name was, but I know they brushed it on & allowed it to stand for a short time before they would put on their first coat of plaster.

we used a thourough bond glue from american gypsum co. by the way, did ya get pm i sent ya with info?

I think your mix of lime/moulding plaster had a little too much lime. A 50/50 mix would get harder and be stronger. It would also set very quickly. Maybe too quickly to work much area at once. Also it can't be retempered once it starts to set.
When you speak of feathering do you mean putting new finish plaster over previously painted plaster and trying to work it down to nothing? Bad idea. When you patch plaster always try to keep the old work clean and never try to feather onto it. The paint will not let plaster bond and eventually it will come loose and leave a ridge where the new work ends and the old work begins. What is best is to stop the new work against the old and keep it flush right there. You can't help getting a little new mud on old painted plaster but never leave it there Keep troweling it off and leave it flush between old and new.
Lath should not be necessary over block. There might be lath at angles and as you said where there is not a substrate to plaster on.

When you say you sprinkled alum on the wall was it in solution? Maybe you only set the outer surface of the gypusm plaster and it was not set undereath. I have only seen this once but what might have happened is that in trying to finish over unset plaster you disturbed the set material and lost the bond that you should have had to the plaster. If the gypsum plaster is still there check to see that it is hard and strong and has set entirely. Dried out plaster or burned out as we say is chalky and a lighter color than set plaster. You can set it with a solution of alum in water but you must do it carefully so that all the lost water needed for hydration is replaced throughout the thickness of the plaster. Sometimes water alone is good enough.

Let us know what you do and how it goes. We like to learn as well as teach.

in feathering, that is what we did , just as you explained. thank you, it sure brings relief to know the problem and the solution. although on some areas i (grated) all th paint away because the old paint was peeling and then applied just the glue and finish(no first coat) and all dried and cracked. But like you said we'll try a liittle more lime in that mix. as for the alum, we just sprinkled it on top of the mixed gypsum on the board and troweled it all up before applying it but this is the first plaster job and we guessed at the amount of alum. thanks again, Sure wish we knew what we are doing , but have learned alot since checking out the forums
prodrywall
p.s. If anyone is ever in our area stop by and check it out (quite interesting)

Yeah I did get you pm Arch, I think sent a reply, but not sure. If we ever get away from here and down thru that way will definitely stop by. Hope this project doesn't break ya man. Good luck!

Tightcoat is our resident pro on all things plaster here on the forum, you're in good hands.