Repairing old lath and plaster wall

gardeninggirl1October 2, 2012

My husband is getting ready to repair a water damaged lath and plaster wall. He has removed the damaged plaster and all the loose plaster brushed out. He has purchased the bonding agent also. Our walls are a rough plaster and we of course want to match it. What do we buy to match the old rough plaster. He bought something but he says that it is smooth plaster and will not match the rough walls that we have. Please help us!!

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I'd say that the final look depends more on the final steps than the type of plaster used. Any type will work to bring the area level with the surrounding areas...the crucial step is duplicating the texture before the plaster surface has set completely.

Experiment with various tools to achieve the look, then use that technique for the last step.

    Bookmark   October 2, 2012 at 10:55PM
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If you have sandy-feeling plaster, all you have to do is mix some masonry sand (sharp sand) into the final coat.

It's a multi-step repair process. First you fill the bottom layer with the "scratch coat", which is scratched with something to make grooves for the next layer. Let it set up and dry

Second coat brings the hole up almost to the top. Make sure this one is parallel to the wall and it gets lightly scratched.

Final coat - finish coat, is where you duplicate the texture of the wall you are patching.

Practice on a board until you get the knack of it. If you post a picture of the wall showing the texture, we can help you find the right tools. Usually you can get it done with household tools ... I've used popsickle sticks and curry combs on patches.

    Bookmark   October 3, 2012 at 11:23AM
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I know you're worried about how the top coat/finish will look but you should consider what's underneath that, first.

Plaster is rarely one coat, almost always two, or even better, three. Each applied separately and left to dry before the next is applied. The different coats are made of slightly different materials as they have different purposes, too.

But the first thing to think about is the lath. I assume you are talking about wood lath?

You said you have bonding stuff, but that's not going to be sufficient to stick the plaster to the lath. It's really intended to stick plaster to plaster, at the edges of the repair, which except in really small repairs isn't enough to secure the plaster to the wall.

Plaster works largely because it is mechanically held to the wall when some of the first (deepest-in) coat is squeezed through the openings in the lath and blobs of it form behind it within the wall cavity. When you tore the old plaaster out, you no doubt saw the plaster material behind the lath, that wasan't sloppy work, it was an essential part of the plaster. Through a chemical curing process as the plaster dries the material on the room side of the lath and the blobs behind are permanently bonded together around and through the lath. Subsequent layers of the plaster are bonded to the deepest layer (and to each other when there are multiple coats) by a reactivation of the chemical reaction as they are applied and then dry. But the strength of the wall is based completely on the integrity of the first coat's attachment to what's behind it. The blobs are called "keys" and in a mechanical way they "lock" the first coat plaster tight to the lath.

The strength of this layer comes because a chemical reaction occurs when the plaster dries (cures) slowly enough to become rock-hard and tightly stuck to the rough surface of the lath. Originally the wood lath (like all new wood) had enough intrinsic moisture content that it did not suck the moisture out of the drying plaster prematurely which would interfere with the vital curing process,thereby weakening the plaster. However when you are dealing with old lath (that was original to the wall), which is now almost tinder dry, you can have a problem. One solution is to try and re-wet the lath by spraying it to the run-off point repeatedly over several days just before starting the repair.

You can also strengthen the repaired-section's adhesion to the lath and to the wall by attaching over the wood lath a section of expanded metal lath. Cut it to roughly fit the space and screw it to the wall studs. (Don't tighten the screws down so much that the metal lath is distorted and pulled down into the spaces between the lath strips. You want it to be a flat, secure plane lying just on top of the lath.) Do not use nails which can cause additional plaster pieces nearby to crack and come off. Very small repairs (6-8" holes) can get by with just attaching (withwires, or careful stapling) the expanded metal lath to the wooden lath itself, but anything larger needs the metal lath attached to studs.

Your first coat of plaster is a rough coat and its pupose is to get good adhesion of the plaster to the wall so it gets troweled onto (and through) the lath pretty firmly. It should remain completely below the surface of the finished wall and there's no need to make it more than nominally flat. It is usually a much coarser mix and slitght roughness on its surface helps subsequent layers stick to it. Only scrape off big lumps which would interfere when the next layers are smoothed on. The first layer is allowed to dry and another one or two coats are added on top to fill the remaining space and bring the repair up to the same plane as the wall. Each will be slightly different in composition; the final one being more fine, and/or in your case more textured.

As recommended above I would definitely try as many trials of your finished texturing as necessary before doing your wall. You needn't used multiple coats while you are practising the top coat texturing. Just try the finish samples on scraps of sheet rock over and over until you've got the technique for the final coat down to a satisfactory match. Top coat plaster sets up quite quickly and you want to be able to apply it, work the texture and be done before that happens.

Before starting this job I recommend doing some more research on working with plaster. There are instructions in books on old house repair and from time to time in magazines that deal with old houses. You should also know that depending on the age of your house modern plaster available for repairs and orginal plaster are not the same thing. However for a small repair I think I would use modern materials rather than trying to source and work with older formulas.



    Bookmark   October 3, 2012 at 2:02PM
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Liriodendron, could you mention specific product names of the modern plasters you are referring to if you were only going to use one for patch repair? Thanks

    Bookmark   October 6, 2012 at 12:19PM
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Use Durabond and Easysand in your choice of hardening time.

Do NOT use 'patching plaster' or 'plaster of Paris.'

Both set way to fast to be useful without a LOT of retarder added.

The classic plaster wall in the US was lime putty-plaster mix.
Europe used straight lime putty as plaster for a long time.
It takes months to 'cure.'

A mix of lime putty (takes weeks to months for curing) and plaster (takes minutes to harden even with retarder added) was used in the US.

The combination takes about 24-48 hours to cure.

Easysand and Durabond are setting type drywall compound that harden by chemical reaction, just like lime putty plaster.

Durabond is as hard as lime putty plaster and for all intents and purposes cannot be sanded.

Easysand is softer and can be sanded if you need to.

The original plaster was NOT sanded, but worked smooth with steel trowels.

    Bookmark   October 10, 2012 at 3:55PM
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