What graywings said. Just paint the whole ceiling. It will be faster and easier with a better outcome than repairing the divot you're making.

Don't create more problems. Just repaint the ceiling. Water stains? Two coats of Zinsser's BIN (red can) on the stained areas before you paint. Or BIN the entire ceiling. Otherwise, you'll probably need two coats of ceiling paint.

We considered that. Problem is that the leaks are in 3 different and large rooms. That would be a massive paint job. The paint they picked is unfortunately not a standard white ceiling paint. Some yellowy cream color. I realize it won't be a perfect match, but best compromise if I can get a section out to match.

Any ideas on how to cut the board?

Oscillating tool,

Or drill a series of small holes and use an old chisel, flat blade screwdriver, gasket scraper, etc. to connect the holes and pry out the sample.

Need dust mask, eye protection, and an assistant to hold a shop vac nozzle close to where you're cutting to collect the as much dust as possible.

Then, we're going to want to see some high-res pix when you're finished patching & spot painting.

Massive paint job...just how 'large' are these rooms? How high is the ceiling?

You will end up making a mess of your ceiling. Doubt it is plaster board. It is a full plaster job in a 1950s house. Make holes in it, and it will keep crumbling. Its not like sheetrock. Fix the hole you made, and paint the ceilings that need painting. I suspect the reason its creamy color is that is is probably white paint that has turned color with age, and that the ceiling really needs to be repainted. Seal all leaks with bin or shellac, and repaint the ceilings. It is not that time consuming, and doesnt all have to be done in one day. You should also be aware that 1950s plaster can and usually does contain abesteos. Please do not poke around in it. Plaster is a wonderful surface but is rarely done anymore because it is very expensive, and hard to even find a trade plasterer anymore. Because of the lack of plasters by trade and the ease of sheetrock use, plaster is just rarely done anymore. Take good care of the plaster you already have. When you hang things in plaster, drill a very small hole and use wall anchors to hang your pictures, etc., just do not go around pounding a nail into a wall, as it will crumble, and you will end up with a hole.

Forgot to mention--

If you opt to remove a section by drilling holes, make the removed section an irregular shape. Don't make it a square, rectangle, etc. Will be easier to get an inconspicuous patch on an irregular shape.

My BIL bought a house that was built in 50's. I think the older builders were sceptical on not using lathe and plaster and experimented a bit with techniques. My BIL walls are drywall but with a very thin very hard coating on them (not a textured finish, it's smooth) seems almost harder than cement. Possibly troweled on after drywall went up. He had to special order a drill hole cutting thing that can cut cement he needed to do some rewiring/adding outlets. A simple drywall handsaw was useless.

On another note. Friend bought house that had double drywall throughout. He was told builder 1940,50, not sure) Didn't trust the new way of doing walls without lathe and plaster.

Izzy, that isnt sheetrock under the 50's plaster. It is called rock lath. It took the place of regular lath that was in the older houses. Older houses also had horse hair for binder in the plaster. 50's houses rarely had horse hair, unless someone actually insisted on it. Some houses (mine did), had 2 layers of the rock lath, before the coats of plaster. My house was located in Minnesota. Well, guess, it is still located in Mn. Just me that moved on, the house stayed there. LOL

Rock lathe was common in the 40's as a base coat for plaster. It looks like sheetrock, but usually came in 4'x4' squares, and has a much more porous finish that let the plaster adhere better.

Bruce

Re paint match, paint stores have sample strips, get a few of those vs cutting into your ceiling.

Okay okay. I won't try cutting it. But I also can't paint it all. I will match as best I can and live with the eyesore until we repaint. Thanks all.

Hi, If you are trying to remove water stains put some bleach in a spray bottle and spray it on a small section and see if it works for you. I have removed a lot of water stains that way success rate about 85%. Make sure to cover items to prevent over spray.

Good Luck Woodbutcher