wall - what is it?

Old 10-23-00, 12:41 AM
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I've just begun to repair some cracks in a ?plaster? wall. i'm not sure what the walls have been made from, any ideas?
medium grey, similiar to concrete
crumbly texture, backed by horizontal lengths
of wood 10mm apart.

Old 10-23-00, 06:06 AM
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Sounds like a plaster wall. You can patch over cracks with black vinyl plastic screen wire in 6"-8" wide strips mudded in just like sheetrock tape. Good Luck!
Old 10-23-00, 09:16 PM
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"Wood lath" plaster walls used wood strips nailed to the studs to form a surface on which the plaster was stuck to. In the 1950's, wood lath was replaced with "gyproc lath" where 2 foot wide by 4 foot long pieces of what looks like crude drywall were attached to the studs and the base and finish plaster coats stuck onto that. This "gyproc lath" was subsequently refined with smoother paper on the front, contoured edges and a larger size and became the drywall we use now.

Crumbly plaster? The only time plaster comes apart easily is when it's been water damaged and has subsequently dried out. In that case, remove everything that comes off without putting up a fight. You're a lot better off replacing more plaster than you are sticking good plaster to loose plaster.

For the base coat, phone around to the drywall wholesalers in town and ask for something called "Pearlite Admix Hardwall" from the Geogia Pacific company or competitor's equivalent. This is what you can use as the base coat to stick to the lath.

For the top coat, I'd go with a gypsum based plaster patching compound, and the most popular one in the world is probably Synko ProSet 90, which has recently undergone a surgical bag transplant, and now comes in a blue bag (at least in Canada) with the wording Synko ProSet 90 LITE SAND on it. I think you can get it in premixed tubs too.

NOW, whenever you're patching plaster, it's a good idea to paint the edges of the existing good plaster with a 50/50 mix of white carpenter's glue and water. This dilute glue will get drawn into the dry plaster and will glue all the sand 'n stuff together as it dries, forming a hard, sticky surface for the new plaster to stick to.

Adding some white carpenter's glue will make both pearlite and gypsum based plaster patching compounds both stickier when wet and harder when dry. However, I've never had great results adding it to the pearlite. The pearlite is pretty sticky as it is, it dries hard enough as it is, and more glue just makes it shrink more as it dries, causing it to crack up pretty bad.

Hold a bright light near the wall when you both spread the plaster and sand it smooth. The critical lighting will exagerate the roughness of the wall, giving you a better idea of where to add plaster and where to sand. Pearlite can be smoothed when dry with a "rasp" tool, which consists of a piece of what looks like cheeze grater metal mounted in a handle. Stanley makes these and Home Depot sells them. You don't really need to smooth pearlite because you normally just cover it with plaster patching compound.

I've been repairing plaster like this for years, but if you ever come across any good web sites that explain how to mix real plaster from sand and dehydrated lime, I'd like to know about that site. I think all plaster is repaired with gypsum and pearlite nowadays.
Old 10-28-00, 11:57 PM
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Beg to differ with some of your advice which I find mostly right on. I think you'll find that if you contact any compound or plaster manufacturer their advice will be to never laminate any paint between layers of coumpound or plaster, or any combination of the two. It will weaken the bond.

Also, there are many reason why plaster will crumble. New paints are one. The old bonding agents are not suitable for many of the chemicals available today. Another is in aggregate formulas of the past. Some just eventually break down. The old lime/marble dust plasters that are considered infinate (marmorino's) were never commonly used in residental work in this century. They are still available, but are quite expensive compared to typical lime plaster. Don.
Old 11-04-00, 05:13 PM
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Sorry I forgot to post this before, hope its not to late.

Should you decide to re-plaster with the old lath, you renew the moisture content of the lath before applying your basecoat. All the moisture has been drawn out. Drape towels and keep them wet with a spray bottle or something similar.

Nestor, not much difference between Gypsum and lime plasters except for slaking. Curring times are very different. Lime can take up to a year, which means no paint or wall treatments for that time. Gypsum usually takes no more then 3 weeks. Lime plasters are seldom damaged by water. When properly prepared, they can be used for pools. Also, I know of the "gyprock" lath being used as early as the 20's. Just some history FYI. Any questions, post em.

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