How do I know whether my walls are plaster or drywall?

Knock on them. Plaster gives a hard, dull, solid sound with almost no give and it hurts your knuckle a little. Drywall sounds hollow and drums between the studs. You can also look at a light switch: pull the cover plate and if the wall section in the opening is close to an inch thick with wood strips behind it, that is lath and plaster. Half an inch of white gypsum with grey paper is board.

Why does my patch keep coming back through the paint?

Almost always one of two reasons. Either the crack is movement and was filled without tape bedded into it, so the joint reopened along the same line, or the surrounding plaster is loose and the patch was skimmed over a field that is still flexing. The fix in the second case is not more compound. It is fastening the plaster back to the lath first, or cutting out the dead area, and then patching.

Can you just skim the whole wall smooth instead?

Sometimes, and on a badly crazed wall it is genuinely the cheaper answer. A full skim coat gives a uniform surface and removes the guesswork of matching an old texture. What it also does is erase the hand troweled finish that gives these rooms their character, so we ask before we suggest it rather than after. On a bedroom nobody notices we would skim. In a formal room with original moldings we would not.

My ceiling is peeling in flakes. Is that a paint problem?

Probably not the paint you are looking at. In Craftsman houses of this era it is often calcimine underneath, a chalky distemper coating that was never meant to be painted over. Modern paint bonds to the calcimine, the calcimine lets go of the plaster, and the whole layer comes away. It has to be washed back or sealed with a primer formulated for it. Skipping that step guarantees the problem returns.

The house was built before 1978. Does that change the job?

Yes, and you should insist that it does. Paint from that period can contain lead, so nothing gets dry scraped or dry sanded open in the normal way. We test first, and where the result is positive we use wet methods, containment and proper waste handling, or we leave the layer sealed and undisturbed under a bonding primer. Most of the housing stock in the older Pasadena districts falls into this bracket.

Do the Santa Ana winds and the dry heat affect the work?

More than people expect, and mostly on the drying side. Hot dry air pulls water out of a patch at the surface while the core is still wet, which is how a repair ends up cracking or a topcoat flashes dull over it. In a bad wind week we slow the coats down, keep windows shut on the exposed side and let the compound cure rather than sanding on schedule. Exterior work stops entirely once wind is driving grit onto wet paint.

What do you actually stand behind afterwards?

Our own workmanship on the repair and the coating, for two years. If a patch we made opens up or a coating we applied lifts, we come back and make it right at no charge. What we cannot warrant is new movement in the building, a roof leak that wets a wall from behind, or a repair somebody else did that we painted over at your request. We will say clearly at quote time which parts of your job sit in which category.

How long will my rooms be out of use?

A cosmetic repaint of one room is usually two to three days. Add board repairs and it becomes four. A room with a genuinely delaminated plaster field can run a week and a half, because setting compound, a texture match and a topcoat each need their own drying window and there is no way to stack them. We schedule around whichever rooms you actually need, and we hand the house back clean each evening either way.

Interior Painting, Pasadena

Your Walls Are Plaster. They Do Not Patch Like Drywall.

Name: Lemuriapop
Address: 1720 Green St, Pasadena, CA, 91106, US
Telephone: (626) 974-4505

Most of the houses we paint in Pasadena were built before drywall existed. The repairs, the fillers and the primers are all different, and a crew that treats plaster like a sheet of board leaves you a patch you can see from the hallway.

Call (626) 974-4505 Ask for a Wall Survey

We tap the wall before we quote
Setting compound, not lightweight filler
Texture matched by hand

Paint roller and tray ready for interior wall work in Pasadena, CA

Notes on Old Walls

What we find behind the wallpaper in Pasadena houses, and what it takes to make it right.

Plaster Is Not Drywall, and Patching It the Drywall Way Is Why Your Repair Came Back

September 1, 2026

The most common conversation we have on a Pasadena job is not about color. It is standing in a hallway with a homeowner looking at a patch somebody made a year ago, which has come back as a hairline shadow or a slightly different sheen or, in the bad cases, as a piece of wall now sitting loose in a hand.

The patch was not badly made. In most cases it was made competently, using exactly the method that works on modern walls. It failed because the wall it was made on is not a modern wall, and the two are far less alike than they look under a coat of paint.

This is what the difference actually is, how to tell which one you have, and what changes about the repair, the filler, the primer and the schedule once you know.

What Is Behind Your Wall

If your house went up before roughly the late nineteen forties, your original interior walls are almost certainly lath and plaster. Thin wood strips, the lath, were nailed horizontally across the studs with a deliberate gap between them. Then plaster was applied wet in three passes.

The first pass, the scratch coat, was pressed hard enough to squeeze through those gaps so that it oozed out behind the lath and curled over as it stiffened. Those curls are called keys, and they are the entire reason the wall stays on the building. The second pass, the brown coat, leveled everything out. The third was a thin, hard finish coat, often lime based, troweled to a smooth or floated surface. Total thickness comes out around seven eighths of an inch of solid, brittle, surprisingly heavy material.

Drywall is a different idea entirely. It is a factory made board, gypsum core in a paper wrapper, half an inch thick in most residential work, screwed straight to the studs. The joints between sheets are taped and covered with compound and that is the only wet work in the whole assembly. It went mainstream after the war because it is faster, lighter, cheaper and needs no skilled wet trade on site.

Both make a flat wall you can paint. That is where the similarity stops.

How Each One Fails

Drywall fails at its weak points, which are its manufactured features. Seams open up. Corner beads dent and crack. Screws and nails work loose and push a small round bump out through the paint. Water swells the gypsum core and turns it soft. Every one of those is local, visible and fixable in place.

Plaster fails structurally. The keys behind the lath are the first thing to go. Decades of building movement, a few earthquakes, someone slamming a door ten thousand times, and those brittle curls snap off. The plaster does not fall down when that happens. It stays right where it is, held by friction and by the keys around it, and from the front it looks like a perfectly good wall. It is not attached to anything anymore.

That is the condition that ruins the well made patch. Compound goes over a hollow area, sands beautifully, takes paint, and then the unattached field flexes half a millimeter the next time the house moves and the repair opens along its edge.

Plaster also cracks in a way board does not. A fine web of hairlines across a whole wall, called crazing, is usually just the finish coat aging and is genuinely cosmetic. A single crack running diagonally away from the top corner of a door or a window is the frame moving and the wall telling you about it. Those two look similar to a homeowner and take completely different repairs.

How To Tell Which You Have, in Two Minutes

Rap on the wall with a knuckle. Plaster is hard and dead and gives almost nothing back. It is a solid, dull sound, and it stings a little. Drywall is hollow and drums between the studs, then goes quiet where you hit a stud.

If you want to be certain, turn off the power at the breaker and pull the cover plate off a light switch or an outlet. You are looking into the edge of your own wall. Plaster shows a thick section, most of an inch, often with the edge of a wood lath strip visible. Drywall shows a clean half inch of white gypsum with grey or white paper on both faces.

One more clue: run a hand across a wall in low light. Old plaster is rarely dead flat. It has a gentle waviness from having been floated by a human being, and that is not a defect.

What Changes About the Repair

Fastening comes before filling. On plaster, the first move on any hollow area is to reattach it. Plaster washers, which are thin perforated discs, get screwed through the plaster into the lath or the stud behind and pull the field flat again. Set slightly below the surface, they disappear under the finish coats. Where the plaster has crumbled or the lath itself is broken, there is nothing to pull to, and the area gets cut back to a sound edge instead.

The filler is a different product. A standard lightweight joint compound dries by evaporation, sands like butter, and shrinks. That is fine on the shallow work drywall asks for. On a deeper plaster repair it shrinks away from the edges and sinks below the plane of the wall, so the repair telegraphs through the paint. Plaster repairs of any depth want a setting type compound, which cures chemically on a fixed clock and shrinks very little. It is harder to sand, which is exactly why it holds.

Depth has to be built up, not bridged. Where a drywall patch is used to fill a hole in a plaster wall, half an inch of board sits well below seven eighths of an inch of plaster. That gap gets shimmed or built up in a base coat, not spanned with a thick lump of compound. The junction between a board patch and an original plaster field is the single most common place we see a shadow line in an otherwise good repaint.

Texture is matched by hand or not at all. Original finishes in these houses were made with a trowel and a float. Sand float leaves a fine even grain. Skip trowel leaves flat islands with texture between them. A spray hopper will not reproduce either, so we match by hand, on a sample first if the finish is unusual, and we judge the match dry rather than wet.

The Primer Question, Which Is Its Own Trap

Even a perfect repair will show if it is painted straight over. New compound and new plaster are far more absorbent than the surrounding wall, which has been sealed by decades of coatings. The topcoat soaks into the repair and dries duller than the paint beside it. Same color, different sheen, and the eye picks it up instantly in raking light. Spot priming every repair fixes it and takes about ten minutes.

Old walls have three other primer problems worth naming. Glossy oil painted plaster and trim need a bonding primer or a real deglossing before waterborne paint will hold. Water stains and tannin bleed from old fir and redwood come through latex indefinitely and need a stain blocking primer rather than a third coat of finish. And on Craftsman era ceilings there is calcimine, a chalky water based coating that new paint sticks to very well while it lets go of the plaster beneath. That is the peeling in sheets people describe on the phone, and the answer is to wash it back or seal it with a primer formulated for it.

Why the Local Conditions Matter Here

Pasadena weather makes plaster work slower, not faster, which surprises people who assume hot and dry is good for drying. Hot dry air, and especially a Santa Ana wind off the San Gabriels, pulls water out of the surface of a patch while the core is still soft. The face skins over, you sand it because it feels ready, and the material underneath is still moving. A week later there is a fine crack across the repair.

So in a wind week we slow the coats down, keep windows shut on the exposed side of the house, and let compound cure through rather than sanding to the schedule we drew up.

The older districts also predate the 1978 lead rules. Testing before disturbing paint is not paperwork here, it is most of the housing stock. Where a test comes back positive, the work moves to wet methods, containment and controlled waste handling, or the layer stays sealed and undisturbed under a bonding primer.

What This Should Change About Your Next Quote

When you get painters in, ask two questions. Ask them what your walls are made of, and ask what they intend to do about any hollow areas. A crew that knows the answer will have tapped the wall while walking through and will already be telling you which rooms are original and which are later work.

If the answer is a shrug and a price, the number is a guess, and the parts of your house that need the most attention are precisely the parts that guess does not cover.

Call (626) 974-4505 and we will come and survey the walls before anyone writes down a figure.

Read the full article

Lemuriapop is a house painting crew in Pasadena, and the thing we deal with more than any other is the difference between plaster and drywall. Tap a wall in a Craftsman off Michigan Avenue and it answers back hard and solid. Tap the same height of wall in a garage conversion done in the eighties and it sounds hollow. Those two surfaces take different repairs, different fillers, different primers and different amounts of time, and the paint that goes over them only looks the same if the work underneath was matched to what was actually there.

Plaster in these houses is three layers built onto wood lath. A scratch coat was pushed through the gaps in the lath so it curled over and locked in place, a brown coat leveled it, and a thin lime finish coat went on top. The whole build is close to seven eighths of an inch thick and it is hard, brittle and heavy. Drywall is half an inch of gypsum in a paper wrapper, screwed to studs, taped at the joints. When plaster fails it is usually because those curled keys behind the lath have broken off and a section of wall is now hanging on nothing. When drywall fails it is a seam, a corner bead or a nail pop. A repair aimed at the wrong one of those is wasted money.

That is also why the estimate matters more here than the paint does. Half of what we get called out to look at in Pasadena is a patch somebody else did last year with lightweight joint compound over a loose plaster field. It went on fine, sanded easily, took paint, and then the wall behind it kept moving and the whole area came back. We would rather spend an hour walking your rooms with a light and a knuckle, telling you which cracks are cosmetic and which are structural, than quote a number from your driveway and discover the truth once the furniture is out.

The wall gets tapped, not glanced atWe work along each wall by hand and by ear before pricing anything. Hollow, drummy plaster gives itself away in seconds, and that survey is what separates a two day room from a two week one.
Setting compound and plaster washersLoose fields are pulled back to the lath with washers and screws or cut out to sound material, then rebuilt with a setting type compound that hardens by chemical reaction instead of drying and shrinking.
Sand float and skip trowel matched by handOld plaster was finished by a trowel, not a hopper gun. We match sand float and skip trowel patterns with the same tools, dry, then check the match in raking light before any primer goes near it.

What Plaster Repair Adds Compared to a Drywall Patch

These are per room ranges for a standard bedroom or living room, walls and ceiling, and they assume an occupied house with furniture to work around. The reason the top tier is so far above the bottom is not greed, it is that rebuilding a delaminated plaster field and matching a hand applied texture takes days of drying time nobody can shorten. We put a firm figure in writing after the wall survey. We do not run a finance product, and we do not want a large deposit either: a small amount to book the dates, then a progress payment when the prep is signed off and the balance at completion.

Sound Plaster, Cosmetic Cracks Only$450 to $780

Hairline cracks filled and sanded
Spot primed, two finish coats

Get a number

Board Repairs and Mixed Walls$720 to $1,300

Drywall patches cut, backed and taped
Thickness shimmed where board meets plaster

Get a number

Delaminated Field Rebuilt$1,100 to $2,400

Washered or cut back to sound plaster
Texture matched by hand, then color

Call now

Plaster, Lath and the Repairs Drywall Never Needs

The work list for a pre war Pasadena interior. Some of this has no drywall equivalent at all.

01Loose Plaster Re-Keyed to the Lath: Where the keys behind the lath have broken, plaster washers and screws pull the field flat against the wood again before anything is skimmed over it. On badly delaminated areas we cut back to a sound edge instead, because a washer only works where there is still something for it to hold.
02Structural Crack Repair: Cracks that run diagonally off a door or window corner are movement, not surface. Those get opened out, bedded with mesh or paper tape into a setting compound, and built in coats. A crack that is simply packed and sanded will reopen along exactly the same line, usually within a year.
03Drywall Patch and Texture Match: Later additions, garage conversions and post war rooms are board, and they get board repairs: cut a square, back it, screw a patch, tape, three coats, feather wide. Where board meets original plaster there is a thickness difference to shim, which is the joint most crews get wrong.
04Calcimine and Old Coating Removal: Many Craftsman ceilings here carry calcimine, a chalky water based coating that no modern paint will bond to. It has to be washed off or sealed with a primer made for the problem. Rolling latex straight over it is how a ceiling ends up peeling in sheets a few months later.
05Primers Chosen Per Surface: Fresh plaster patches suck primer unevenly and flash through the topcoat, so they get spot primed. Old oil painted trim and glossy plaster need a bonding primer. Water stains and tannin bleed need a stain blocker, not another coat of finish paint. Three different problems, three different products.
06Interior Repaint With Containment: Sanding plaster and compound makes fine dust that will travel through a house in an afternoon. Zippered doorways, floors covered and taped, vents sealed for the duration, vacuum extraction on the sander, and the rooms left usable at the end of each day.

Eight Questions About Plaster, Patching and Texture

How do I know whether my walls are plaster or drywall?: Knock on them. Plaster gives a hard, dull, solid sound with almost no give and it hurts your knuckle a little. Drywall sounds hollow and drums between the studs. You can also look at a light switch: pull the cover plate and if the wall section in the opening is close to an inch thick with wood strips behind it, that is lath and plaster. Half an inch of white gypsum with grey paper is board.
Why does my patch keep coming back through the paint?: Almost always one of two reasons. Either the crack is movement and was filled without tape bedded into it, so the joint reopened along the same line, or the surrounding plaster is loose and the patch was skimmed over a field that is still flexing. The fix in the second case is not more compound. It is fastening the plaster back to the lath first, or cutting out the dead area, and then patching.
Can you just skim the whole wall smooth instead?: Sometimes, and on a badly crazed wall it is genuinely the cheaper answer. A full skim coat gives a uniform surface and removes the guesswork of matching an old texture. What it also does is erase the hand troweled finish that gives these rooms their character, so we ask before we suggest it rather than after. On a bedroom nobody notices we would skim. In a formal room with original moldings we would not.
My ceiling is peeling in flakes. Is that a paint problem?: Probably not the paint you are looking at. In Craftsman houses of this era it is often calcimine underneath, a chalky distemper coating that was never meant to be painted over. Modern paint bonds to the calcimine, the calcimine lets go of the plaster, and the whole layer comes away. It has to be washed back or sealed with a primer formulated for it. Skipping that step guarantees the problem returns.
The house was built before 1978. Does that change the job?: Yes, and you should insist that it does. Paint from that period can contain lead, so nothing gets dry scraped or dry sanded open in the normal way. We test first, and where the result is positive we use wet methods, containment and proper waste handling, or we leave the layer sealed and undisturbed under a bonding primer. Most of the housing stock in the older Pasadena districts falls into this bracket.
Do the Santa Ana winds and the dry heat affect the work?: More than people expect, and mostly on the drying side. Hot dry air pulls water out of a patch at the surface while the core is still wet, which is how a repair ends up cracking or a topcoat flashes dull over it. In a bad wind week we slow the coats down, keep windows shut on the exposed side and let the compound cure rather than sanding on schedule. Exterior work stops entirely once wind is driving grit onto wet paint.
What do you actually stand behind afterwards?: Our own workmanship on the repair and the coating, for two years. If a patch we made opens up or a coating we applied lifts, we come back and make it right at no charge. What we cannot warrant is new movement in the building, a roof leak that wets a wall from behind, or a repair somebody else did that we painted over at your request. We will say clearly at quote time which parts of your job sit in which category.
How long will my rooms be out of use?: A cosmetic repaint of one room is usually two to three days. Add board repairs and it becomes four. A room with a genuinely delaminated plaster field can run a week and a half, because setting compound, a texture match and a topcoat each need their own drying window and there is no way to stack them. We schedule around whichever rooms you actually need, and we hand the house back clean each evening either way.

Pasadena Blocks Where the Walls Are Still Plaster

We work across Pasadena and the towns either side of it, and the older the housing stock the more of our week it takes.

Call (626) 974-4505 and tell us roughly when the house was built. That one detail changes the estimate more than the square footage does.

Pasadena, CA (91101, 91103, 91104, 91105, 91106, 91107)
Altadena, CA
South Pasadena, CA
Sierra Madre, CA
San Marino, CA
Eagle Rock, CA
Arcadia, CA

Have Us Tap the Wall Before Anything Is Quoted

The survey is free and it takes about an hour. We work along your walls with a light held flat and a knuckle, mark what is hollow, tell you which cracks matter and which do not, and give you a number for the version of the job you want rather than the version that flatters our invoice. If the honest answer is that your walls are fine and you only need paint, that is what we will write down.

Call (626) 974-4505