Now that all the loose plaster areas on my sloped study ceiling are secured back to the lath using the Big Wally's PlasterMagic® system, I can reflect on what I've accomplished, how I'd do it differently if I could go back and do it over, and give some highly gratuitous advice on how you can do it, too, probably with better success:
1) The day I saw the cracks (back in 2007!), if I couldn't repair it right away, I'd put up bracing to keep it from getting any worse.
2) If I didn't brace it when I first noticed the cracks, I'd certainly brace it before I drilled One Single Hole.
3) Rory B's probably got this in the directions someplace, but if you suspect a large section of ceiling plaster is losing its grip . . .
a) First find out how thick your plaster is. You can tell somewhat by feeling the "jump" your 3/16" masonry bit makes to the lath once it gets through the plaster. But if that feels like working blind, get out the big guns, like a ½" masonry bit, and drill a hole big enough for you to see.
b) Then take your 3/16" masonry drill and drill a hole maybe 3' or 4' or so away from the crack or exposed lath edge. Does it penetrate only the thickness of your plaster? Good. But don't be complacent. Drill a couple more test holes in line with it, parallel to your crack. If they're all good, move in 6" and try again. When you find your drill going in 1/8" or more past the thickness of your plaster, that's where you start.
4) Next you drill your field pattern. The directions on the conditioner bottle say that should be an 8" to 12" square grid, avoiding the spaces between the laths, of course (mark them with a pencil and try again).
(Me, I went for overkill and drilled mine at 6" x 6". May explain why I went through so much of the adhesive so fast. But my plaster is thin, only 3/8", and I wanted to avert cracking.)
5) After that (is your bracing up?), drill your edge holes, about 1½" from the crack or exposed lath, 3" o.c., which should hit every other lath if you're going perpendicular to it.
6) Note that No. 4 & No. 5 are S.O.P. for this system. And take as read the part about vacuuming out the holes, using the conditioner, squirting in the adhesive, etc., etc. But for your saggy ceiling, be sure to begin screwing in the plastic washers at the point farthest from the crack or edge. This will bring your plaster up gently and gradually and go a long way to prevent further cracking and collapse.
7) I readhered some smaller cracked pieces around the edges. This works if your plaster is thick. But at 3/8", they cracked further and look really bad. I'm hoping-trusting-praying that the final skim coat will cover them over, but right now, I feel I should've pulled them off and left a clean edge.
8) The directions say to leave the screws and washers in for 24 to 48 hours. I was paranoid and left them up for twice that long, at least. This doesn't hurt anything. Your drill will back them out just fine.
9) When you pull away or pry off the washers, some of the paint may come away with them. You can correct this when you go over the area with joint compound. But you may, like me, discover that the paint layer is very thick, very plasticky, and very loose (the plastic factor may help explain why the plaster failed in the first place). In that case, I'd carefully peel it away until you get to a place where it's good and adhered. No point in trying to achieve a nice smooth surface over loose paint.
10) About surface protection and clean up: Yes, you can wipe off drips with warm water and a sponge. You can scrap off hardened adhesive with a putty knife. But the water in your bucket will get filmy, and despite your wiping, you will end up with a shiny film on your ceiling or wall surface. This doesn't matter in your target area, since it will need repainted or repapered anyway. But for adjacent surfaces and objects . . .
a) Remove them if you can from the field of battle. Never mind if it's an annoyance and a pain. Do it.
b) For anything that can't be moved, know that newspaper or contractor's kraft paper will not suffice. The Big Wally's conditioner is made to penetrate lath and plaster. So you think it won't soak right through that paper? It certainly does, and I've got the newsprint glued to my painted wall and trim to prove it. Use intact, heavy grade plastic or plastic-backed dropcloth instead. No rips or holes allowed. And tape it continuously along its top edge, especially if you're repairing a ceiling. Otherwise, that stuff will find its way behind it and underneath, believe me.
c) If things goof up (don't they always?) and a scraper won't do to get the hardened conditioner or adhesive off, I've found that a little Ivory® soap and elbow grease on one of those green abrasive kitchen sponges works in time to get it off. Or you can use Goo-Gone® for items (like my 1950 black-with-gold-leaf Singer sewing machine) that won't bear scrubbing. Though you might try the scraper again on those.
Reflecting on Item No. 6, I used that method with the plaster remaining on the sloped part of my study ceiling, and it worked well. I did not think of doing it before that when I tackled the loose plaster on the flat part of the ceiling, continguous to the sloped ceiling failure site. I saw it cracking, too, and wanted to get it secured right away, before I started the sloped ceiling repair. I began drilling in my washers right at the cracks, with the result that the plaster went all pillowy and pitted. I didn't even get it up to the lath sufficiently and had to go back and try bringing it up again, after I'd explored further out and discovered that the sagging area was more extensive than I'd realized.
But maybe it wouldn't have mattered regardless of where I put in my first washers. When I started peeling away the loose paint in this area, I found that the plaster above it was crumbly and lacked all integrity. A strip of heavy paper joint tape was revealed, showing where some previous owner had repaired the place before. I don't see joint compound or spackle adhering to any of this crap, and even if it did, it'd be an irregular mess.
I vacuumed up the plaster mess yesterday. But when I can face this again I shall have to come back with the drop cloths and tape and demolish every last bit of bad plaster on the flat ceiling repair area, clean it all out, and replaster the gaps. I hate doing plaster overhead, but it's not as if I can't do it or haven't done it before. I have the pictures to prove I have, after all.
"When I can face this again" is the operative term. I have a house guest coming in two months and now that (please, God and Rory Brennan!) no more plaster should be falling down, I'm going to stop out of the 3rd floor plaster work to concentrate on getting the 1st and 2nd floors liveable before she comes.
I only hope there aren't any more areas of ceiling failure lurking . . . Couple days ago I buttoned up some plaster on my 1st floor hallway ceiling, around the perimeter of the stairwell, and found that the loose areas went much farther from the cracks than I had any reason to expect. Paranoia strikes deep . . . I find myself drilling exploratory holes in random places in the third floor ceiling . . . and getting nervous about rain on the roof . . . and worrying that my dog's strident barking will set up dangerous vibrations à la Memorex . . .
But then I reflect that the reason I fear plaster falling is the very reason I don't want to tear it out and put up drywall. So I'll just keep watch. And keep the Big Wally's handy.