# Forum Home Renovation Plastering  HELP !! How To Repair LARGE cracks in lath and plaster walls

## ivan_351

Hi Guys and girls , just need some information!! 
A old double brick home with solid plaster inside the previous owners just covered the walls with supposedly modern wall paper back some 30 years ago ?? 
I have removed the paper and cracks in the plaster all over the place. Understandably this was expected. I have read,  " How To Repair cracks in lath and plaster walls and ceilings" on  http://www.how2plaster.com/restlath.html which is a great source of information. 
The problem starts here , i have removed one of the floor skirtings to access the power point so the electrician can can run his wiring to the other side of the room where a another power point is badly needed rather than running a cord across the room. 
When the skirting boards was remove large chunks of plaster separated from the wall also. 
The plaster is very thin about 2mm to 4mm and the base seems to be lime based mix.which in some areas is up to 20mm to 30mm thick. The lime base just falls apart to dust in ones hand if pressures is applied on the broken piece of solid plaster. 
I tried in one spot a full mix of corners cement which seems to have bonded but have been now waiting for well over 4 to 5 days for it to set . It is still wet to touch once I have sanded a layer of to smoothen out the patch work. If I do the whole house this way I might be waiting years for it to set ( Completely dry ). 
I have repaired cracked sections in the method described in the link I posted and the corners cement has set well and the repairs look and fee magic. 
My other thought is what can I use for a base ( not the old lime method ) and have this base just a few millimetres below the original surface level and then finish it of with corners cement and the works done. 
A gentleman mention something call , " plaster sand " and a concrete cement mix of 6 parts  sand and 1 part concrete cement and apply this on the walls where large sections have cracked badly and fallen out. 
I went to Bunnings today and asked for " plaster sand " and the guy looked at me if i had two heads and said there is no such thing? 
What are people using to apply as a base for their solid plaster repair when large sections are needs to be replace. 
What are you using for the top coat as the plaster finish ? 
Can someone shed some light on this topic would be appreciated  
Cheers

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## myla

hi, 
currently fixing one up at the moment 
a.  take off lime mortar & plaster until i get a reasonably solid feel, unless small wall and strip all 
b. bunnies have bags of wash sand which is found near cement or go to sand yard for better value 
c. use 3 sand, 1 cement, 1 lime and a bit of bondcrete 
d. nail chicken wire onto studs first and also poke mortar out of battens so new plaster gets a key 
e. screed off, so under surface by about 2-3mm, especially where new meets old, middle of wall can be a bit higher but edges you want good 
f. let dry for at least 7 days in this climate 
g. i normally use cornice cement for smaller areas, or Boral hard plaster with some lime (mixing this is critical or will be hard by the time you take the stirrer out) 
word out

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## Rod Dyson

One of the best solution to large areas I have used, is a mixture of Perlite and cornice adhesive. I used this method extensivly in a huge restoration job at the historical society in the city. When I went back ten years later not one area had cracked nor could the repairs be seen.  
The great thing about perlite is that it bonds well with cornice adhesive and bulks it up so it can be layed on in a very thick coat. When it sets it can be scraped back flat with ease and skim coated with a cornice cement only coat. 
We also use this mix to run cornice institu in curved stairways. Did this in my own home with a circular stair. 8 years late not a single crack. 
Perlite can be hard to get, try a few garden supply places or places like Rudy's that supply renderers products. 
Love this stuff for major crack repairs so easy to use. 
Cheer Rod

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## ivan_351

Hi guys , thank you for the immediate replies  
Myla..... What I can understand the method you using is for solid plastering on wooden battens walls. I'm needing to repair large sections that have cracked and pulled away from the double brick walls. The house is all double brick. 
I need to mix up some form of mortars to apply a base coat to the brick work and then finish with a plaster coat to finish off the surface. 
I need a recipe for the best mortars mixture that will do my base coat to fill in the large sections and cover the brick work as if I'm doing a rendering job. 
Can one apply a mortar render as a the base for the solid plaster similar to what the guys do in rendering prier to adding the finished render texture on a house external and or internal walls? 
What is a good mortar render mixture consist of by ratio or weight in sand , cement , lime , water ? What band names products work the best ? 
Ok I supposed different mortar mixture strengths for different applications??? 
Myla thanks once again for you input. 
rod@plasterbrok .....had to look up and see what "Perlite " is ? so did some yahoooooooooing and seems like a great product with some very interesting properties and highly used in the building industry. I hope this is the right *Perlite* your referring too.   *Perlite* is not a trade name but a generic term for naturally occurring siliceous *...* The distinguishing feature which sets *perlite* apart from other volcanic glasses *...* http://www.perlite.net/  *Perlite* is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing..... http://en.wikipedia.org/wiki/Perlite  
What ratio mixture of "Perlite and cornice adhesive " do you recommend to use for the base for the solid plastering repair.?  
When your referring "cornice adhesive", I assume your referring to something like CSR Gyprock cornice cement 45 ? 
Also do you apply any washing compounds and/or bonding compound on to the brick work ? or just a simple scrape clean and use a coarse bricks' broom/brush to prepare the brick surface ? 
Guys sorry with all the silly question , I would just like to get the right materials the first time so I can complete the repairs before I paint the room.  
rod@plasterbrok thanks once again for you input. 
Cheers

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## myla

hi, 
yes same mix, if have to build up then dont use a "bag" of render, 
so: 
3 parts wash sand
1 part lime
1 part GP Cement
bit of bondcrete (like 100ml to 5lt say) 
best 2 fill in two stages, then scrape back if you like 
i normally brush wall with only a little h2o just prior to coating 
thanks

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## Rod Dyson

I use about 50/50 ratio. 
Prep work brush away dust and wet surface before application. 
Cheers Rod

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## ivan_351

Hi , thanks once again guys for your help. 
Ok got hold of " Perlite " gessssssssss the stuff is not easy to find other than hydroponics shops who want a arm and a leg for small bags. So after making numerous telephone calls around the traps to my surprise not many people knew what the product was, finally someone pointed me in the right direction where to find the " Perlite ". *Sage* *Horticultural*. 121 Herald St. *Cheltenham* VIC 3192. TEL: (03) 9553 3777 www.*sagehort.com.au* has "Perlite" in 100lt bags and only weighs 7 to 8 kgs. Will give this ago this weekend. 
Also bought a second 20kg bag of cornice adhesive nearly finished the first bag might indicate how badly the solid plaster walls are cracked and this is so far only the hallway and the front room , one 20kg bag of cement and two 20kg bags pre_mix mortar from Bunnings , the specs on the product are no where to be found other than it is recommend for brick mortar , paving , wall render and so on. The actual cement to sand ratio is not specified mmmmm. There is a telephone number for technical department so will give them a call. 
Will experiment with mortar and Perlite mix and see how this renders works onto brick work for a base preparation for the solid plaster large section for repairs. 
Also will try the Perlite and cornice adhesive mix method.  
cheers

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## Rod Dyson

Ivan Call me if you have any difficulty.  Be happy to discuss it over the phone.  BTW where in Melbourne are you? 
Cheers Rod BTW my phone number is on my web site.

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## rrobor

Not quite sure but when I was researching building a Pizza oven I think I found out that the easy source of pearlite was kitty litter,. Dont know if you want gravel in your plaster but I would suspect it might  be OK in a base coat. I though stand to be corrected as what seems OK on paper could be a disaster on the wall.

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## ivan_351

> Ivan Call me if you have any difficulty. Be happy to discuss it over the phone. BTW where in Melbourne are you? 
> Cheers Rod BTW my phone number is on my web site.

  Ok thanks for that Rod will keep the offer in mind  .....otherwise over the weekend had a go with Perlite and cornice adhesive  mix at a ratio 50/50 works great increases volume of the mixture substantially. Very lightweight. Had to experiment a few times with different water content. Small batchers were mixed up as we all are aware the cornice adhesive sets within about 45 minutes and as it approaches the set time it gets harder and harder to apply and work with. 
One thing one will notice using this cocktail mixture of cornice adhesive and perlite it is a bit more tricker to finish the surface smoother on small crack repairs  but for large section it was awesome to use. As one applies the scraper across the surface one picks up the fine perlite granules and they start scratching the surface finish. Applying the scraper 90 degrees to the crack solves that problem or just scraping small surfaces at a time and cleaning the scrapper clean on each application. 
The combination of perlite and cornice adhesive is great combination for solid wall plaster repairs specially in areas where the plaster has become totally detached of the wall and a new section is needed rather than just doing a glue job on the section to retain it to the surround wall. While still sounding hollow when one knocks on the wall section that's damaged. I just replace these types of section with new cornice adhesive and perlite mix, I'm sure the repair will out live me now. 
Head down bum up more work needs to be done , now I dream and eat cracks oops LOL 
cheers

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## piscean

If anyone gets stuck for perlite try the gardening section in Bunnies, i buy it from time to time for making my own cactus and propogating mixes but they always have it down there at my local B, it's sold in bags for hydroponics.

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## Rod Dyson

Use the perlite mix for the base coat only use straght cornice cement to do a thin second coat over then top coat to finish. 
Cheers Rod

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