I have been plastering over painted walls and ceilings for over twenty years We use plasterweld By Larsen and one coat of base coat plaster , with one coat of finish plaster, polished or textured as the case may be. We seal the tinted walls with a wax or tung oil depending on the color and sheen desired.Sealing prevents fingerprints and makes them washable. We use natural colors and dyes when posible Robin

You say that the drywall is breaking down. It normally won't break down unless it has gotten damp or wet. I think before you cover anything up you should determine if there is a problem. If there is correct if first. The plaster loosening up is fairly common. It could be for a number of reason, but I think you should still be sure that there isn't moisture problem there too. As far as covering the existing walls without removing the old stuff.....I have personlly covered I would guess 40 or 50 rooms and ceilings over the years without tearing out the old stuff. Is there insulation in the walls and will you ever need it there? Celulose can be blown in from the inside and, of course, that would be done first. If the walls aren't in too bad of shape I go over everything with 3/8" drywall and use lots of screws. Where you are going over plaster and lathe you will sometimes hit studs, but often just the lathe. The screws into the lathe will hold as long as you use lots of screws. Over the drywall you will have to locate and hit the studs with screws. Also on going over the drywall, you will have to end the pieces over a stud just as if you were applying it to bare studded walls. If you end it between studs there will be nothing to hold the edges and the job will be ruined. When I go over existing walls I make sure that any wiring that might be needed is done first. If necessary I make holes to run wire. If there are any real uneven spots on the wall I shim out the low spots with scrap pieces of 1/4" or 1/8" luan or paneling. I don't remove the door and window casings or the baseboards. I run the drywall right up against the casing and fill in any gaps with mud when I am taping. Now a lot of remodelers won't agree with going ovre the existing, but I've never had a problem as long as I've used plenty of screws that are long enough to penetrate the drywall or the plaster. If the walls or ceilings are terribly uneven you might have to use 1/2", but I've always used 3/8".

If you still have the wood lathe to fill the hole and just need plaster, you can use plaster of paris (most like your old plaster), or something like versabond drywall compound to fill the areas. The old plaster is probably fairly thick, so you could also screw or glue in some 1/4" or 1/2" drywall first to fill the void, and then use plaster or drywall compound on top of it to completely fill in the area. Plaster of paris will dry very hard, so gradually layer it in to minimize the need for sanding. This is one case where a little is better than a lot, because you can always add more layers. You can use drywall tape on the edges where it meets the old plaster, but may want to score the old plaster so that you can embed the tape a little. Plaster of paris also has a very short working time (about 10 minutes), so don't mix up too much at a time. Drywall compound will probably match almost as well as plaster of paris, and its available in mixes that are easier to work with (20 minute set time, several hour set time, etc.). Bruce

Are you sure what you removed was plaster and not mortar? In either case, it's not really clear what you want to do. If you want to have the brick exposed and visible then you have a sizable job in front of you replacing the mortar by tuck pointing the joints- after removing any crumbling mortar bit by bit - where it has failed or been removed. This is quite a bit more than a "cosmetic repair". The mortar is what holds the chimney together. If you want to coat the entire surface with something there would seem to have been little point in removing what was already there. You would still need to do some fairly major repair work on the chimney before proceeding as well. Honestly and without wishing to be unduly alarming, the photos show a chimney in dire need of evaluation by a mason or other professional. By chipping off whatever was present, you may have seriously weakened the chimney. The mortar is in very poor condition and I would guess that there are many loose bricks. The expanding foam product shown in the bottom photo provides no strength and is unsuitable for repairing masonry. A coating of some sort and some paint is not going to fix this.