Need your advice on lime plastering please

[Socksitis]

Hi,
I have started out with some lime plastering, but need to work out where I may have gone wrong. Just filling out some holes to make it levelish before the levelling coat. Used NHL3.5 at 3:1 I had painted the old lime render underneath with DG27 as it was crumbly but sound. There are some cracks in the patches this morning - what have I done wrong and how can I make the next job better? Is it about the mix itself, perhaps it was a bit too wet? Is it about pushing onto the wall, should I have gone over it with a wood float?
Just learning, very keen, so need guidance please - yes I have been on a course, but it is all about putting it into practice and I do believe, like all things, everyone has to start somewhere and the only way to get better is to keep at it.

 

[Chris-Burgess]

If it's just patching old plaster, I use gauged plaster. This will make the lime plaster set quicker. It will still carbonate. With patching I would have taken it back to the substrate wet it down and filled it out with gauged coarse stuff with hair in it. Then rub it up with a wooden float to counter shrinkage slightly cut the edges back to allow for the setting coat which should be damped down first,gauged 50/50 putty and plaster quick scour with the float and trowel finish. Wet the a edge of the patch with water, clean edges with a sponge to wipe off excess plaster. Run a joint rule around the patch edges to get flush finish.

 

[Damon]

Is this indoors or outdoors?

If indoors I'd Personally I'd be using a fatlime (lime putty) for inside with a 3 sharp sand to one lime mix (by volume not weight) rather than the NHL. I'd normally add some horse hair into the mix for the first coat also.

Make sure you dampen the background surface enough as it will crack if it dries out too quickly. Some cracking is normal and expected however and the larger cracks can be "plugged up" before carrying on with your second coat.

Edit: just re read your post. It could be that you used a too wet mix. You want it to be as stiff as possible whilst still being workable. Like I said above though some cracking/crazing is normal.

 

[MikeG]

Everyone assumes sand & lime mix, but it was often originally chalk rather than sand. Chalk doesn't shrink, whereas sand, if it has any clay element (and most do) does. My old place had no sand used in any of its mortar, render, or plaster.

 

[Damon]

Problem is with chalk is that it's not as readily available as sharp sand and I would guess far more expensive. I'd therefore only really see a practical advantage of using chalk over sand as a topcoat where the finish is important.

 

[Zebra]

Did you float it while it was still green? I find a damp car sponge better than a wooden float, but you have to compress it before it dries and cracks.

 

[MikeG]

Problem is with chalk is that it's not as readily available as sharp sand and I would guess far more expensive. I'd therefore only really see a practical advantage of using chalk over sand as a topcoat where the finish is important.

No, but the pre-mixed products, including hair or fibre, and sometimes other fillers such as insulants, are readily available.

 

[Flyfisher]

Everyone assumes sand & lime mix, but it was often originally chalk rather than sand. Chalk doesn't shrink, whereas sand, if it has any clay element (and most do) does. My old place had no sand used in any of its mortar, render, or plaster.

Agreed. I've given up using sand-based plaster in favour of chalk and have found it far, far easier to use. It can also be applied quite thickly where required, which is ideal for uneven 'rubble' walls such as we have in many places, and can therefore be much faster than traditional three-coat systems.

Check out this further information: http://www.anglialime.com/products/wet-mix-plasters-angliamix-mortars-renders/fibrechalk

Problem is with chalk is that it's not as readily available as sharp sand and I would guess far more expensive. I'd therefore only really see a practical advantage of using chalk over sand as a topcoat where the finish is important.

Yes, it might be more expensive but not, I'd say, particularly so in the grand scheme of things. As we're talking DIY here the largest part of the cost of plastering has already been eliminated, ie the labour. Then there's the convenience and 'mess' factor - have you the space for loads of sand and bags of lime? And a mixer? Then there's all the cleaning up to do afterwards. Compare that with buying some ready-mixed tubs (or large bags for bigger jobs) and suddenly a large part of the work is already done for you, and very consistently too.

Coverage is about 1m2 per 1mm thickness per litre. So a single 15 litre tub will cover about 5m2 to a depth of 3mm. Not bad, I think, for about £17 - less if you buy in bulk.

The only downside I've found is that the finished result and colour is attractive enough on its own that the urgency to further decorate is reduced to the point where it may not get done for some years. You have been warned! :wink:

 

[Socksitis]

I have done it! - so pleased, went over with some Heritage lime putty and it is lovely - may not be the most even of surfaces, but I do not want an even surface, the product was just lovely to work with and I even managed to get some nice rounded corners - thanks to the sponge. Just lovely to touch. I wonder, does the sand feel go away? It feels like the surface is sand that comes away - perhaps it dries differently?

 

[urowho]

Heritage lime putty and what, neat, fine sand or chalk?

 

[MikeG]

Chalk, and fibre.

It's simplest to buy it ready mixed.

 

[Socksitis]

Ready mixed