Rendering - how to get a flat finish?

[Nigel Watts]

I have just spent many hours rendering a small length of garden wall. Initially I had terrible problems getting my NHL 3.5 lime mix to stick to the underlying render layer which I think must have been sand and cement. By Saturday evening I eventually got all the render on and put in my fake ashlar lines.

The next morning I looked at my handiwork as was not happy. The render was far from flat and the ashlar lines only drew more attention to this fact. I had been using a cross grained float try to get a flat surface but it had not been enough. I then spent the bast part of Sunday morning correcting the problem. I achieved a much flatter surface but only through much work and constant checking with a straight edge and spirit level, again using the cross grained float to distribute the render after it had dried a little. When I put in the ashlar lines I noticed that some parts were still slightly concave, so some further adjustment was made. When cleaning my tools I noticed that my wooden floats were badly warped - the cross grained one especially was quite convex.

This experience has made me wonder how professionals seem to manage to achieve a good flat finish in so much less time.

My hypothesis is that I was trying to do too many things the same time, namely getting the render to stick, getting it flat/vertical and achieving a good surface finish and that I should have done the work in two or even three separate coats. For the next section of wall, where I have not had the same problem with adhesion, I have applied an initial coat and focused on getting this fairly true. I have also planed my floats to remove the distortion.

 

[LadyArowana]

Professionals usually make anything look simple, whether it's doing perfectly round pizza bases, handmaking noodles, tailoring a suit or teaching a Zumba class. They have the correct tools and they specialise in doing the thing they do over and over again. The problem with DIY is that you do things only a few times, and by the time you learn how to avoid the worst mistakes you have finished the job. For a pro there may well be small variations and challenges for each client, but they have their basics down pat. I'm sure you have made it look lovely though as you are always so attentive to the details.

 

[Nigel Watts]

Just checked the wall this morning. Some cracks have appeared in one section and when tapped the render sounds hollow. So much for my plans for a leisurely breakfast.

 

[LadyArowana]

Just checked the wall this morning. Some cracks have appeared in one section and when tapped the render sounds hollow. So much for my plans for a leisurely breakfast.

Well, don't tap it then!

 

[Zebra]

I have done very little of this myself, but I think that sand and cement won't form a good base, you would have problems getting it to stick, because it doesn't have any suction, and if the surface of it is flat, then it doesn't have any "key" either for the new layer to hold on to. Yes I think the professionals would do it in two coats, keying the base coat with a comb scratch or one made from a cut off lath. I believe, from the lime plastering course I did, that the base coat must have “gone off” so you can scratch it with your fingernail but not dent it with your thumb before applying the next coat. After applying the second coat, ideally you'd come back and “float it up” after about 2 hours with a wooden float. If too soon, it’ll move the aggregate about and mess it up. If too late you can spray it and do it. This closes any cracks and flattens the surface. The risk with over trowelling at the time it goes on, is that you'll bring all the lime to the surface and weaken the layer. Then you'd sponge it the next day.
You could try sponging the layer you have now with a dense type car sponge. This can remove any superficial trowel marks, and may even hide the joins between extra bits which you add in now to fill in the concave bits, but I wouldn't guarantee it!
If the worst comes to the worst, try scratching in a diamond pattern into the layer you have, and apply another coat! Hope this helps in any way.

 

[88v8]

Just checked the wall this morning. Some cracks have appeared in one section and when tapped the render sounds hollow. So much for my plans for a leisurely breakfast.

Well, don't tap it then!

Doctor, doctor, it hurts when I do this...
Then don't do it...


Here's how to get lime to stick to cement blocks, allegedly. May help. I think it was previously flagged on PPF.
http://www.stastier.co.uk/nhl/guides/renblks.htm

Ivor

 

[Zebra]

Yes, I remember that link. Of course, there are blocks of different densities and suction, which that link doesn't seem to differentiate between, although it does recommend an additive if the background has poor suction, which your sand and cement render probably resembles most.
Of course one of the reasons for the two coat approach is that it then matters little if the base coat cracks.

 

[plasticpigeon]

Hello Nigel,

I have nearly plastered my kitchen with render, but I haven't done the finishing coat yet. I have learned a few things though. I have managed to get a reasonably flat surface in 1 coat, but a half inch of plaster/render is prone to crack, and the unevenness of the walls in my house means that there is massive variation of thickness, so I have found it easier to do 2 coats, the first is there to stick it on and to even out the large irregularities and the second to straighten. On large areas I have found setting up render screeds (lines of render every few feet to use as a guide) before the second coat very helpful. I have found all layers need rubbing with a float to consolidate the layer, even it up get rid of any cracks. There is a best time for this when the render has hardened up a bit so won't get dragged about, but it still soft enough for it to be redistributed. When applying the plaster I have found that moving the trowel over the surface horizontally as well as vertically helps get the render on a bit more evenly. On the second coat I have used straight pieces of wood to drag across the surface using the screeds as a guide to get it straighter before rubbing up with a float. Plastic floats are really cheap, and don't warp (have you been soaking your wooden float before using it, it needs to be well soaked so that the side that is rubbed on the plaster doesn't get more wet than the side with the handle which leads to the float warping to a convex shape) so I have been using plastic ones. If you want a finish that is slightly rough, you could use a sponge float rather than a sponge to finish, this does a surprisingly good job of redistributing the render even more, and also gives a rougher finish a bit more like stone. Sharp sand from a builders merchant has really big stones in so I have found plastering sand better. Lastly try a mix substituting for NHL with a 50:50 mix of NHL and hydrated lime. I have used this to point up below the dpc in a few parts of my house and it has gone hard, so will still set in damp places. It is much much easier to use as the hydrate makes it much more pliable and putty like. I honestly feel a bit cheated by companies selling NHL as I don't believe it is as appropriate for restoration as they suggest. I have rebuilt a few areas of my place with it and really wish I hadn't. I am convinced that 3.5 is too hard for building, pointing and especially plastering/rendering as it is so hard. I have found it as hard to remove from a few places as a cement mortar and nothing like as soft as the original mortar. It goes so hard that I wonder if it is just a lime cement mix, so I can only imagine NHL 5 is even more like cement!!!

 

[dans]

Also limited expertise, but I have just had the whole of the inside and outside of my timber framed house lime rendered and we have removed a lot of cement, so I've become familiar with both materials over the past year!

So, from my limited expertise, lime does need something rough and generally porous to key onto. It's fine on timber which has been keyed, brickwork which is not smooth (eg old reds) and even modern block work because of its rough texture. It won't though hold a bond on smooth cement. If you can't get all the cement off, just knock off what you can and do a scratch coat in lime, then finish over the top.

Also found the grade of sand important too. Too finer sand on both coats does not hold well, we've found.

This is simply from my limited experience. Where's Middi when you need him !

 

[Lime]

I think the link makes the most important point that the first coat should be sprayed or cast (thrown) onto the surface and not disturbed with a trowel/float.
It doesn't matter if the first coat becomes crazed or cracked, it is the key to the substrate that is important.

 

[jocelyn plummer]

Nigel, if in doubt, blame it on the weather conditions :roll:

 

[Zebra]

What did you do, Nigel? Have you managed to rescue it?

 

[Penners]

If the substrate is cement, are you worried about the breathability of your finish coat? If not, then you could use the technique that modern plasterers would use, in order to get the finish coat to stick on the cement coat. Apply a wash of dilute PVA to the substrate, and mix some PVA into the finish coat.

I'll now go and wash my mouth out with soap.

An oft-used technique for getting rendering flat is to fix temporary "grounds" to the substrate. These are vertical strips of wood, the thickness of your intended final coat. You'll make life easier for yourself if these grounds are tapered in section so that the surface against the wall is a bit narrower than the surface facing you. Fix them 1 to 2 metres apart and then render the first "bay", leaving your mortar slightly proud of the grounds. Once you have rendered the whole bay, use a thing called a floating rule or a feather-edged straightedge to cut back the rendering so that it's flush with the grounds. Lay the floating rule across the bottom of the two grounds (it must of course be longer than the width of your bay) with its sharp edge at the top, and work it slowly upwards with a left-right sawing action.

Repeat this process for all of the bays. Allow the rendering to stiffen up a little, but not to go off.

The reason for tapering your grounds now becomes apparent. Undo their screws and lift them carefully out of the trenches that they have left in the plaster - the taper makes this much easier. Now fill the trenches with render and carefully trowel level with the bays on each side. You can use your floating rule (or a shorter one) one for this, but you need to be very careful not to damage your previous work.

Finally, when the surface has firmed up sufficiently, start rubbing it over with a wooden float (I prefer one with slightly rounded edges) in circular strokes, to consolidate it. Splash it with a bit of water if necessary, to work up a bit of a slurry on the surface.

Well - that's what I'd do, anyway. An expert may do it differently.

 

[Nigel Watts]

Lots of great advice - thank you.

On taking off the render in the defective area I was very surprised to see that I had added an inch or more in some parts to get it level and vertical. The substrate I was applying it to, and which I had assumed was itself level and vertical turned out to be anything but - so much for professionals getting it right! The moral of the story I think is not to assume the thing one is rendering is already correct.

I applied a quarter inch or so layer this morning and sprayed a it two or three times during the day, which has been warm and windy. On inspecting it again this evening a crack had appeared in one place, which sounded hollow. I sprayed again and trowelled it very firmly. I am keeping my fingers crossed.

I had no idea one was supposed to soak wooden floats before use.

 

[Louis Henry]

A flat finish? How boring! I did my own lime plastering because I was afraid a professional plasterer would have got it too flat, not matching the original lumpy bumpy hairy finish. I do quite a good lumpy bumpy, though I say it myself.

 

[Nigel Watts]

A flat finish? How boring!

This is a wall between my front garden and the pavement which would originally have been rendered flat and marked with lines to imitate ashlar like the front of the house which has classical detailing. Rustic lime work would not have been in keeping here.

The other moral of the story is not to rush when it comes to lime. I was in a hurry because we are oping the garden to the public in a fortnight and thought I could get away with a single coat.

On checking it again this morning it seemed OK.

The point about harling is a good one which I did not pick up earlier. Ideally I think I should have harled, then applied two coats.

 

[tobydog]

I watched the Mike Wye video on lime plastering and they made harling look very easy and accurate. In reality, on the first few attempts, it looks as though it will knacker your wrists and cover everything but the wall :roll: . Definitely the best way, in my very limited experience, to get a good key without using artificial means.

 

[Nigel Watts]

A number of years ago I did some harling indoors. I had a chimney breast which had previously been opened up for the insertion of a cooker restored back to a normal sized fireplace opening. The builder used breeze blocks, which I then harled before lime plastering. It was very messy but quite satisfying once I had got the hang of it.

When the outside of my house was being worked on (rendered brick, c 1840) I examined an area when the original stucco had to be taken off. The brickworkbeneath looked very much as if it had been harled before being rendered.

 

[MatthewC]

The moral of the story I think is not to assume the thing one is rendering is already correct.

Or you could delete "the thing one is rendering" in that sentence and insert "anything to do with older properties".

But then the philosophical question is what does "correct" mean in this case? My excuse for anything out of true in this old house is that it "adds character", especially if I did it. That's why I love this place rather than some modern rectilinear architectural block.

 

[Penners]

you could delete "the thing one is rendering" in that sentence and insert "anything to do with older properties".

That's often true of medieval buildings, but with a high quality, London, Georgian property, one would perhaps expect a more refined approach to flat, square, level, even... etc.