I'm going to replace the exhaust fan in the bathroom, and I need to enlarge the hole.

This is a 1951 house, with plaster, not Sheetrock.  Actually it's plaster board but this stuff is hard on cutting tools.

I think when I did the original I used a jigsaw and a lot of blades.  This time I want to use my sawzall.

Searching turns up a Milwaukee plaster blade, but it isn't in stock, I think it's discontinued.

I need something I can buy at HD or Lowe's

Here Toy Go.

I used to do tons of reno's, as a professional carpenter. Use an oscillating tool, not a sawzall, to cut lathe & plaster.

I was going to suggest a Milwaukee Torch metal cutting blade, if it'll do 1/4 steel it'll prolly cut plaster and lathe, but it looks like HDepot carries a plaster specific blade

https://www.homedepot.com/p/Ivy-Classic-6-in-6-TPI-Bi-Metal-Reciprocating-Saw-Blade-Cuts-Plaster-5-Pack-28318/310212617

I’d recommend a cut off wheel.

If you are going to use the sawzall  I would wear  a respirator as well.

So a blade for cutting wood and metal will cut through plaster (no lath here)?

As for the cutting wheel suggestion?  Nope.  I made that mistake years ago opening up a closet.

It took years to get the plaster dust out of the room.

I think because of the location, a sawzall is the best tool, with the right blade of course.

Dont use sawzall  , either a jigsaw or oscillating saw. Cut off wheel work too ( dust is just part of it )

If you insist on sawzall, get short demo blade, like from Diablo

I used a jigsaw to cut the original hole.

The plaster just made the blade smooth as a baby's butt in just a couple of inches.  I destroyed a bunch of blades

This is tough stuff from the 50's

6 3/8 carbide hole saw?

Lenox Carbide reciprocating saw.blades.

Learn how to start a jigsaw or Sawzall blind.
The Sawzall is easier to hold at a very shallow angle for a shallow cut.
While you can pivot either into the wall a Sawzall lends itself to an almost parallel to the surface cut with very little overrun into the cavity better.
The lower powder production is much better than the wider rotary blade and higher speed.
Have a helper follow the cut using a Shop-Vac and there will be very little dust.

The rotary tools spray a huge cloud that is hard to contain well.

With the Sawzall you can make a cut that barely goes into the cavity to hit wires and cables.

A Double insulated or battery tool is NOT a good idea if you may hit a power cable.
You WANT that ground to help trip the over-current on the circuit.

If you do it enough times you WILL eventually hit a shallow cable in an old house.

To be clear, I am enlarging an existing hole.

From 9" to 10.5"

Open the hole as much as you can and check for wires and cable.
This is one of the reasons to start the hole with a hammer.

Hit just hard enough to punch through so you can look around the area.

Just about any Sawzall blade will rip though the thin expanded metal used in plaster walls.

My dad's 1934 house has plaster on wood lath with expanded metal at inside corners and
metal bead with expanded metal at outside corners.

I got very good at minimizing excessive damage.

I know repair with Easy-Sand, but before it was widely available use the hard stuff and learned hot to tool it smooth.
I got even more lessons later on when I hired a retired Plasterer.

Those plaster walls never saw any abrasive in creating them.

The brown coats have plaster, lime, and animal hair.
The grey coat has plaster, lime, and sand.
The thin finish coat is around 1/8 inch of lime plaster.
It takes weeks to harden off but tools easily and smooth.

I still  have all the plaster tools. A Darby, Steel square trowels (Use mason's finishing ones to get a better size).
Mason's Finishing Trowel

The modern "plaster trowel" (often around 4 in. x 6 in.)  with the tiny handle is to small to work correctly.
The small handle makes it hard to make large sweeps and raise the leading edge.

A mason's polishing trowel has enough size to cover large areas quickly, and the large handle allows you
to take sweeps left and right by barely tilting the blade up on the leading edge.

The "Plaster's Handbook" has some reprints from the 1971 to 1984 (those dates are in my copy)  and gives all the detailed instructions.
Even that is now over $70 at Amazon.

It is a large multi-step process.
This was a reprint with a title tweak
Plasering Skills

Still over $70 with only a few copies at Amazon.
Used copies starting around $32.
Well worth having if you need to do even somewhat historic reconstruction.

I have not looked recently, but The US Park Service likely has detailed standards and instructions for true 'historic renovations."
They keep track of that kind of stuff for when truly historic places require renovation.

Shredded fiberglass insulation is an excellent substitute for Horse and Pig Hair.
It is a lot of work to get that coat (plaster, lime, and fiber) on with correct keys though the lathe.
Then rough it up for the gray coats of plaster lime and sand.
That needs to be smoother but perfection is not required, yet.

The gray coat of plaster, lime, and sand has to be reasonably smooth but only within about 1/16 in of smoothness.

The top coat is lime putty with very little plaster to speed the setting.
The long setting time allows for plenty of work time to smooth it out with larger and larger leveling tools.
The 3 to 4 foot Darby comes into its own  on this layer.

Lime alone takes days to weeks even in the thin layer.
It sets by absorbing CO2 from the air  leaving you with that snowy white 'flint hard' show surface.
Tooled to a smooth and flat polish.

After the first pass you mist it with water and make a final pass with large steel trowels to a smooth polish.
Not being able to sand that top coat puts a real premium on getting it smooth before final set.

Most sawzall blades have a hook to the teeth, which can hook in plaster. The one posted above is more like a jigsaw blade, just triangle teeth, and should work well

Here is the best option if you are going to do it again- https://www.makitatools.com/products/details/XDS01Z

Bosch (and probably others) make Masonary jig saw blades. They work great for stuff like this.

This

Sawzall vibrates too much you will crack the part you want to
Keep

Ok, I have one of those.

Does anyone make a plaster blade for it or will a metal cutting blade work?

Rotozip?

Winner! Any sawsjigjiz saw is going to beat the piss out of the plaster, go rotary tons of bits that wont destroy shit.  Dremmel works just as good and you can add flutes to the barrel of any firearm!

Tons of fine dust.
If you use a Sawzall you can keep the blade almost parallel to the plaster surface
to reduce vibration in the plaster and the depth of the blade into the cavity.

Cutting into blind cavities has its own set of dangers.

@Avidrook

Project Farm has done a number of videos on sawzall blades.

If i were to use my reciprocating saw for this task I would use a metal blade.

WRONG!!!

HD has a carbide blade that works very well for plaster and any surface with sand in the mix. They were $8 or $10 each last time I bought them, but they do work well.

Thanks, I'll for look for carbide.

Based on past experience, this old plaster board eats blades like popcorn

Jesus, a non segmented diamond blade on a 4.5 grinder holding a shopvac with a bag in it will knock it out in a hurry. Everyone and their mom can float plaster these day's, if you have the patience. When I learned it was an art, with the YouTube video's out there now. Nearly anyone can do it, with practice.

Any blade rated for nail embedded wood will do fine.
Carbide is just not worth the price over a package of steel blades.

Let's see.....

I need to enlarge a hole in the ceiling from 8" square to 10.5" square and so I need to learn to plaster?

You can get glass / ceramic blades for your jigsaw.  Pricey.  They aren't toothed blades, they have diamond grit (or something close to it) embedded in them.  I've bought them at home depot.

I tried cutting hardi panel with them.  Got about a foot or so and broke the blade at the shank.  The cutting surface was fine.  I had a bad angle on the cut.  It probably would have worked if I had set up better.

The reason I'm leaning towards the Sawzall is I think it will be easier to control while standing on a ladder in a shower pan.

I may be wrong, but as I said I cut the original hole with a jig saw and It worked, I thought the Sawzall would be faster and hopefully not as messy.

The problem with jig saws is the vibration perpendicular to the face of the plaster, its weakest direction.
A Sawzall held almost parallel to the face avoids almost all the vibration.
I usually use about an 8 inch or 10 inch long blade and only the first few inches is in the plaster cutting.

With a 10 inch ones when the very tip gets badly worn you can snap it off and use the next few inches.

No reason to run the Sawzall at high  speed either.
Use the low speed range and have a helper follow the cutting
holding a Shop Vac with a crevice tool right at the cutting point.

Dust masks should be worn.
Something a little better than the really cheap 'nuisance masks.'
Plaster dust is hell on your lungs.