Plaster - Advice on patching where lath is missing

Need advice, technical help or opinions, you will find plenty here! (Technical posts here)
1918ColonialRevival
Knows where blueprints are hidden
Posts: 907
Joined: Tue Jan 26, 2016 8:58 pm
Location: Baltimore, MD
Contact:

Re: Plaster - Advice on patching where lath is missing

Post by 1918ColonialRevival »

Wood lath is still available at lumber yards. Big boxes won't have it, but any well-stocked lumber yard will sell it by the bundle. The last time I got some I think it was around $11 a bundle.

Here's what I do in cases like yours. I'll use a 3/8" masonry bit to drill holes into the good plaster surrounding the missing area about 6-8 inches apart and about an inch and a half inside from the break. Drill until you feel the bit hit the lath. I then vacuum the dust out of each hole. I use a spray bottle to moisten the lath and plaster inside of each hole, then inject construction adhesive into each hole. I use a screw and plaster washer to "tighten" the existing plaster back to the lath. Use a screw and washer in every other hole. Let it sit overnight, then remove the screws and washers. Your plaster should be secure.

To patch the missing areas, I'll replace any broken or missing lath with new. I mix up some Structolite, then moisten the lath real good with a spray bottle. I use a trowel to apply the structolite to the lath, making sure to press it into the spaces between the lath to get it to key. Getting a good key to form is the most important part of the project and is the main objective for the first layer. After the first layer goes on, before it dries, use an old comb or similar tool to groove the newly installed layer. This will give a "tooth" for the second layer to grab onto. Mist the new plaster with your spray bottle, mix more Structolite, and do another layer. Try to make this one a little smoother than the first layer and stop about 1/8 inch lower than the finish coat of the existing plaster. As you wrap up this second layer, groove it just like the previous layer. Finally, use plaster of paris or joint compound to make your finish layer.

It sounds like a lot, but you'll get the hang of it after practicing a little. Find some older books on plasters and cements - they will be valuable resources. I don't advocate using drywall. You lose the soundproofing and fire resistance that plaster offers.

historicalwork
Been here a good while
Posts: 229
Joined: Sat Aug 29, 2015 12:01 am

Re: Plaster - Advice on patching where lath is missing

Post by historicalwork »

Really appreciate the additional advice. 1918ColonialRevival, I had saved out a few good post from around the internet and I would say your advice mirrors a lot of what I read. In particular, using structolite for the base and second coat (if doing 3 coats). At the same time, I've seen many that simply used durabond for all costs - some using durabond (brown bag) for the base and then easy sand for second or top coats. Most all seem to suggest using pre-mix for a final skim coat. Oh, and number mentioned adding the metal lath - I had gotten some earlier today even thought I hadn't decided which course of action to take.

I spent a little more time on the wall today. Since I had the plaster weld, I brushed it on all the exposed lath and along the edges of the existing plaster. I also brushed it on some of the areas where the top coat had peeled off. I still struggled a little (putting way too much thought into all of this) with how to make sure I keep the right thickness at the base - especially knowing I had to install the baseboard and ensure it lined up including at the door trim. So, I got a couple furring strips figuring I would screw it in along the baseboard as a guide. I notice that to the left the plaster is maybe 3/8", in the middle about 1/2", and just at the right (where there is a bulge) back to maybe 3/8". I figure I can leave the furring strip in place because I'll be nailing the baseboard there anyway.

As I read articles on plastering and adding up to 3 coats, I was thinking about how thin some of the areas are on the wall. I don't know how I could reasonably build up 3 coats in areas that are 3/8". Based on a random search, seems plaster walls can by around 7/8" to an inch overall. Maybe this wall I'm working on was redone at some point. In any event, It seems like I'l be hard pressed to get a base coat on and then a final coat. Am I missing something?

User avatar
Willa
Revered expert in almost everything
Posts: 1369
Joined: Tue Dec 06, 2016 9:46 pm

Re: Plaster - Advice on patching where lath is missing

Post by Willa »

A video that explains the differences between joint compound and plaster:

https://www.youtube.com/watch?v=CoF5F9AuZ98

Another video that explains step by step about mixing and applying a traditional plaster wall, including using Plaster Weld:

https://www.youtube.com/watch?v=GuZ8wFQIQPU

You will need to take the approach which best suits your needs. A younger house with thinner plaster needs a repair which will match what is there.

1918ColonialRevival
Knows where blueprints are hidden
Posts: 907
Joined: Tue Jan 26, 2016 8:58 pm
Location: Baltimore, MD
Contact:

Re: Plaster - Advice on patching where lath is missing

Post by 1918ColonialRevival »

If the rest of the trim throughout the house sits out about the same distance from the door and window openings, I doubt the walls were re-done. Some plaster installations were done in two coats, though three is what is traditionally recommended. Not sure how old the house you are working on is, but I've noticed that plaster tended to get thinner as time went on, especially from the Depression years through WWII. Wallboard (also known as gypsum board, sheetrock, etc.) was introduced in the late 19-oughts and became more widely used in the 1930s. After WWII, probably at least half of new housing construction used wallboard in lieu of plaster.

Small areas of metal lath for a patch are fine, but I would discourage using it throughout an entire room. It can create a Faraday cage effect and drastically attenuate any RF signals you are attempting to receive in the room (radio, over-the-air television, cell phones, etc.).

historicalwork
Been here a good while
Posts: 229
Joined: Sat Aug 29, 2015 12:01 am

Re: Plaster - Advice on patching where lath is missing

Post by historicalwork »

Our house is a 1905 Foursquare out in the Maryland area. I suppose the issues is more the unevenness of the walls because you are correct that most of the baseboard trim does line up at the doors about the same. I did add some metal lath in a couple spots where I thought it would help - maybe a total of a couple sq ft. Right or wrong, I started out yesterday with the easy sand (45) route. I have two coats on it. In the one area, I feel like it's going to take a couple more coats to bring the surface up for skimming. Otherwise, I'd have to put it on really thick. In other areas, I'm almost there. Did have an excuse to get a better drill - my smaller drill started smoking trying to mix the mud. Also picked up one of those hawks between the coats - didn't think it was needed but it does help. Going to add another coat today.

phil
Has many leather bound books
Posts: 4616
Joined: Tue Aug 18, 2015 6:11 pm
Location: Near Vancouver BC

Re: Plaster - Advice on patching where lath is missing

Post by phil »

I would open it up a bit more. put a box on the opposite side of the stud so the switch plate clears the casing, pull off the casing, repair the wall then replace with another casing that isn't butchered.

to trim the lath, use a jigsaw, it wont' shake the wall as much as a sawzall. cut a square hole and expose the studs, the one you see plus the one the casing is nailed to.

you can cut any lath out of your way using a hammer and sharp chisel where the lath crosses a stud just cut it half way so it is still attached to half the stud, but leave the other half of the stuff clear to accept the 5/8 drywall..

use 5/8ths gyproc, or whatever fits. it is important that it doesn't stick up more than the existing as you can't fix a bulge easily but you can fill a hollow.

you can add more studs or a piece if you want more to screw to, probably don't need to.

trim your new piece of drywall about 6 inches too large for the hole. cut the BACKING PAPER to below the hole size as if you were making X's and O's or a # sign.

snap your drywall and peel away the excess drywall , you will be left with a piece of drywall that fits the hole with face paper that is about 3" bigger all around the hole.

butter the back of the drywall, get lots around the edges, just slop it on the backside.

fit the piece and screw it to the studs. take your drywall knife and smooth the face paper out flat on top of the plaster wall. it will act like a flange. That excess drywall compound is going to form a key just like the plaster does.

when I do this I like to mix about a tablespoonful of Elmers wood glue into my drywall compound. mix it well and use it, but when you smooth out make sure to catch any drips, you can't sand it. If you mix glue in it dries hard like plaster.

after it's dry you can do a little skimcoating to perfect it but the patch will look pretty good already if you do it this way.

If you need to fill big holes with drywall compound , remember if you glop it on thick it will shrink and crack..

so you can get a fist full size clump of plaster of paris. then add enaough cold water until it stops absorbing, then add another two fist size clumps of drywall componnd to the mix.
Mix that up well and use it , it will then dry almost instantly so clean your tools and mixing board or tray.

or if you want you can just use lath and plaster and do it that way. If the casing is to be painted you can plug the hole and paint over it.

if you have too few outlets you could install a 4x4 box and put the switch as well as an outlet in that box if it's a handy location for another outlet.

the knob and tube wire is another thing. I'd pull in new wire myself and ground the box, bring it up to code. you might have to consult an electrician, if you are leaving K and T you might be able to at least switch to ground fault breakers, not my decision how far you take it. I rewired all mine with all new switches and outlets and wire and breakers etc..

this video ( below) shows the trick of making a "pizza" using drywall mud and plaster. its worth watching. You can use this stuff to fill big holes, if you want to.

if your drywall is all cracked you can add 1/4" drywall and do the whole wall, if you want to. but you will have the thickness difference to contend with around window casings etc.
Phil
https://www.youtube.com/watch?v=sp-RatKtLYU

historicalwork
Been here a good while
Posts: 229
Joined: Sat Aug 29, 2015 12:01 am

Re: Plaster - Advice on patching where lath is missing

Post by historicalwork »

I need to get some pictures updated - haven't been back in the room in a couple days (back to work). But, the base coat of USG easy sand is up. So the major areas are built back up close to level. I also had to pull out two small reading lights on another wall - pulled out the old wiring, added new receptacle and box, and patched the two areas where the light figures were at. Had to repair the lath where they put in the boxes for the lights.

I started on the one bad wall only because it was the worse area. I still have some other smaller spots and cracks on other walls to deal with. Also, because I removed the closets, the ceiling fan in that room is not centered. I may move that over - luckily, I believe the wiring comes over from the side to which I need to move it. I also had a small area of wall paper which I took down and it had some rough (but minor) areas underneath I need to patch.

I need to go back to this one wall with the major repair to determine if I need skim coat it all or if I can get by with smoothing specific area. While still not an authentic approach to repair, the majority of articles on skim coating point to using thinned bucket joint compound. If skimming the whole wall, a few recommend taking the roller approach - using a roller to get the joint compound on the wall and then smooth out. It looks like USG does not recommend using easy sand for a skim coat. I hope to make some progress tonight and tomorrow. I'll post up pics. Appreciate the continued feedback.

phil
Has many leather bound books
Posts: 4616
Joined: Tue Aug 18, 2015 6:11 pm
Location: Near Vancouver BC

Re: Plaster - Advice on patching where lath is missing

Post by phil »

sounds like good progress.
When I skim coat I mix the mud quite thin, like pancake batter almost. I use a paint mixer on a drill and mix the whole bucket quite thin. do that the night before and leave it to let the air come out. try not to add too much water, if you do you cant' take it out, youcan always add a little nmore water to the tray but getting it mixed quite thinly with the drill saves a lot of tray mixing.

Use a fairly wide knife. I first load the knife and press it down , distributing the compound everywhere along the path of an arms length swath. for this I dont; care if it is even I just coat the whole surface for my next pull. get some everywhere.
Next I clear both sides of the knife and pick up a small amount and knock the corners of the blade off a bit so more is near the center of the blade, but you want a bead near the front edge of the blade. ( this is so that when you pull back you get less of a line beside the swath, so it doesn't ooze out along the edge.

It'll still leave a little line near the edge of your knife, but just knock that off with a knife when dry as you are sanding the next coat.
Press it down quite firmly and pull back slowly and evenly in one motion ( keep your speed, pressure and angle consistent) picking up more than I leave and leaving the mud behind the knife level, then deposit what you pulled off back in your tray, move over and repeat, then move back 18 " or so ( an arms length)and repeat, doing another swath of long strokes like that..
then a very minor sanding and you can do that two or three times if you want it absolutely perfect.
Yes definitely keep a light at an oblique angle to show the imperfect areas as you are working.
Don't be afraid to get close and sand any high spots off and primer and start again. that prevents you from sanding too much out and gives you a fresh start, otherwise you go sort of snow blind and it gets hard to see what mud is wet and what is dry. the stipple helps grab the next layer of skimcoat.
I don't roller on mud, that wouldn't work at all for me.

after you use most of the tray, Youll find this sort of filters out the fines and you are left with coarser mud. Also as I go I dip my knife in water and give the stuff a good stir to maintain the consistency of mud/water. after practice you'll get to know what is too thin and you will get better at not dropping it.

If you get any hard chunks in it , or if it gets down ti the coarser particles, it will seem less "creamy" throw it out and get fresh mud.
For skimcoating I use the Classic finish type. it is made of finer particles, for finishing.
https://www.usg.com/content/usgcom/en_C ... pound.html

I find a lot of the secret is getting the right consistency and so as I go, the act of putting mud on and taking it back off dries the mud and so to compensate I dip my knife in a bucket of water and continue mixing as I go, to keep the stuff sort of fluid almost. if it's too thin it'll get hard to control the drips. If it's too inconsistent you'll get air pockets, it really needs to be mixed well. when you are tray mixing you'l see the air bubbles in it , keep mixing until you get rid of any bubbles. You'll spend a lot of time tray mixing to maintain the consistency and you need to for it to work right.

when you first start doing this you'll drop more mud, as you get better you'll learn to control it better, takes practice.
when you pull back the knife , if it is more flat to the surface it will deposit more , if it is at a higher angle it is more of a scraping action and it will pull the mud off. it takes practice to get that angle , speed, pressure and mud consistency right. If you don't like how it laid you can just pick up the mud and start that swath over, when you get a nice flat pull , move over. I go left to right and cover the line left by the edge of the blade on the right hand side. You'll always have a little line left by the left side of the blade, at least until you get it more perfect to the point where you are using a higher angle and removing most of your mud. These lines act as bridges when dry as youll sand most of the line off but it will create an area for the knife to ride on on the next coat.

on the next coat you can go the other way if it is a ceiling, alternate directions between coats.
you can pole sand or just use a block. I always sand in circles, it isn't like wood where you sand with the grain.
Phil

User avatar
Willa
Revered expert in almost everything
Posts: 1369
Joined: Tue Dec 06, 2016 9:46 pm

Re: Plaster - Advice on patching where lath is missing

Post by Willa »

historicalwork wrote:I need to go back to this one wall with the major repair to determine if I need skim coat it all or if I can get by with smoothing specific area. While still not an authentic approach to repair, the majority of articles on skim coating point to using thinned bucket joint compound. If skimming the whole wall, a few recommend taking the roller approach - using a roller to get the joint compound on the wall and then smooth out. It looks like USG does not recommend using easy sand for a skim coat. I hope to make some progress tonight and tomorrow. I'll post up pics. Appreciate the continued feedback.


The walls I have fixed have usually not needed the entire wall skim coated. I fixed the worst areas, then feathered the edges out as much as possible. Like an area that had a 6" x 6" patch might wind up with an area that is approximately 30" x 30" to the end of the feathered edge. If the area had inconsistent projection, the feathered area will probably be larger.

I think the roller method is more about replicating the texture of paint stipple to better blend in with the wall texture v.s. an alternate way to apply joint compound ?

historicalwork
Been here a good while
Posts: 229
Joined: Sat Aug 29, 2015 12:01 am

Re: Plaster - Advice on patching where lath is missing

Post by historicalwork »

Willa wrote: I think the roller method is more about replicating the texture of paint stipple to better blend in with the wall texture v.s. an alternate way to apply joint compound ?


There are a few videos / discussions where people suggest rolling the thinned bucket joint compound on the walls versus putting it on using the trowel / hawk when skimming the entire wall. I suppose the idea is getting a good layer on the wall and then spreading it with the trowel. This is a random video off youtube - not saying it's good, right, wrong etc - but this is an example of what I've seen. https://www.youtube.com/watch?v=jeGWbgAUvCM. Having said that, I first going to try as you suggested - feathering in the various areas. I've seen many recommend getting a primer coat on to better gauge the smoothness.

Post Reply