Rendering tips ?

[Nick]

I need to render some outside wall patches, some of which is painted brickwork, then overpaint it all. Is there a site with some tips on how to do bare brick and painted brick The bare brick is extremely dry and like blotting paper - you can squirt a hose at it and the water just disappears as it hits the wall - spooky to look at ! (Unibond it first maybe?) or can someone here enlighten me please.

Questions I know about so far.... no doubt others I may not have thought of...

1) What materials ( ordinary builders sand and Portland cement ?) 2) What mix ( 5 : 1 ? ) 3) Any additives ( PVA or washing up liquid and how much ?) 4) How dry/sloppy a mix ( as dry as possible or wet as possible, or just nice and workable?) 5) If Unibonding the painted bricks, do I apply a coat and let it dry and another and wait to go tacky ? 6) After levelling, how long to wait until "polishing to the usual matt finish", presumably with a wood float.

I have done some plastering but not rendering...

Thanks,

Nick

[marpate1]

Hi Mick,

This may help, but I've never tried myself.

[Chris Bacon]

1:1:6 cement:lime:sharp sand (you can get special rendering sand, but it's hard work).

No. Certainly not washing up liquid for anything!

A little drier than the mix for brick laying.

No, don't. Just apply the PVA 1:5 PVA/water and render on it straight away. Can't you get any of the paint off? It would be better.

Use a wooden or plastic one, let the render go off until resilient to the touch but not hard, then go at it with your float. Be careful of adding too much water.

[Nick]

Hi Chris, Thanks for the tips - all noted I was walking past a house the other day where there was rendering going on ( PVA /washing up liquid containers on the wall) and the carpenter there said that the renderer was using it in his mix as a plasticizer he thought, but didn't know for sure - hence the query.... and he was using ordinary building sand - would this give a smoother finish to sharp sand ? I have several bags of building sand left over so was hoping to be able to use it if possible .....

Nick

[Chris Bacon]

I would not employ this person (the one doing the rendering). The "plasticising" effect of washing-up liquid is great, it makes the muck go on nicely, however that's all it's useful for, and it can be quite bad for the quality of the final result. A proper plasticiser is far, far better, but it's still really only helping with the handling of the stuff when you put it on - the final set render *may* be harder when using plasticiser, though. With render, you don't really want an extra-hard finish, hard enough is all you want. If it's very hard, then the render won't cope at all well with any movement, and may crack off in slabs. That's why

1:1:6 is nice, the lime makes it handle very nicely, and the set is not vert hard (although it is quite hard enough!).

Yes, perhaps - however, it contains a lot of "fines", and is very likely to shrink and crack when it sets. The special sand I mentioned for rendering has a very even grain size indeed, it looks like caster sugar but sandy coloured. It's hard to work, but gives a very superior result. Washed sharp sand is probably the way to go.

Use it for something else, or give it away - don't put it on your walls!

Re. PVA - put it on and render straight over, because if it sets, you may have trouble getting the mortar to stay on the wall - the PVA is supposed to leach into the brickwork *and into the render you put on* - giving a strong bond when set.

[Nick]

Hi Chris

Thanks for the rapid and informative reply - I now understand the reasoning !

I will try and do the job over the next week weather permitting.

Thanks again,

Nick

[Rob Morley]

I'd have thought a wire brush in an angle grinder would make a fairly easy job of that.

[Nick]

Hi Rob,

Thanks for the reply - I have those so will give that a go first.....

I suppose I could always fix some suitable mesh on the wall first to give it a good key...

Nick

[Rob Morley]

EML - expanded metal lath

[meow2222]

We dont know what kind of bricks youve got, nor what type of building. With some buildings rendering in sand/cement is really not a smart idea. Lime render is safe on any building type.

Wirebrushing bricks may do significant damage to the bricks, depending on what type they are. Cement rendering may do likewise, again depending. Use lime.

If you use EML, the common galv EML is not so good, as it rusts and makes a right mess of it all. This doesnt happen with SS, though its several times the price.

NT

[Nick]

Hi, Thanks for the added info - the bricks are in very poor shape approx 110 years old and pretty weathered - I would guess that in their day they were the cheapest you could get, or even rejects..... Part of the wall is already rendered and there was once an outside toilet, which was removed, exposing the area needing to be rendered.

By "damage to the bricks" do you mean cosmetic damage or something else ? Why would it matter if its going to be rendered over ?

In any case, I will take the advice and use Lime in the render

Thanks again, Nick

[meow2222]

ok then you do need to use lime not cement

cosmetic and structural: damaged soft bricks soak up water, freeze, and slowly disintegrate. Eventually the only solution is to rebuild the wall.

because its the bricks that hold the wall up, that are teh wall. You dont want them disintegrating and the wall becoming unstable. Its a very slow process, but thats what cement does to soft bricks.

The advice was not to use lime in the render, but to use lime render. Lime render does not contain cement. Cement based renders have the following problems on Victorian properties:

- they trap water thus cause brick damage

- they adhere to the brick more strongly tha the brick adheres to itself, thus they often damage the brick when they eventually come off

- being impervious they prevent the wall drying out, and increase the chances of damp in the house

Lime render is 3:1 by volume of sand and lime putty. Lime putty is simple to make yourself. It is very slow setting, it takes weeks, and if frost is likely where you are it should be covered over until set.

The bricks were probably fine until someone cement rendered them. Typically bricks were made onsite, with the junk used on the inner leaf.

An angle grinder and wirebrush would do substantial damage to Victorian soft bricks, and would be truly unwise.

If you want more detail on Victorian properties and cement and lime,

[Nick]

I reckon this is exactly what I have got - fortunately its on a sheltered wall that rarely if ever gets / got wet and it can breathe from the inside. - I will be careful in getting a key on the bricks and not lay into it with anything brutally mechanical... the shame / concern is that the wall is already 80% rendered with what almost certainly is a cement render unless it looks identical to lime render...

It sounds like a discussion with Anna Kettle may also be worthwhile for some additional hints and tips... especially with how to make lime putty, which I hadn't even heard of...

Thanks again, Nick

[Chris Bacon]

It doesn't. Lime render is soft. Cement render is very hard. To match what you've got, and avoid problems due to widely dissimilar renderings, use 1:1:6, which is widely accepted.

[Chris Bacon]

In Victorian times? I disagree with that. What makes you think it's so?

[meow2222]

The 2 are significantly different. Lime render is essentially chalk, and ages by gradually wearing down, thus has a self cleaning action. It is also self repairing, it reacts with CO2 to grow hard crystals across any cracks, thus can tolerate movement and repair itself.

Use round corners with lime rather than square, as its not as hard as cement.

Either buy it, or else get a bag of hydrated lime from a BM, dump it in a bin, mix with water to form a paste, and store it either with an airtight lid, or under a layer of water, for a few weeks. Its then ready to go.

3:1 is the usual mix. Dont add any cement.

Lime rendering is easier than cement. It has lots of body, sticks better, and youve got all the time you want to get the surface right. Have fun.

NT

[meow2222]

I guess it all depends on when and where. If its late Victorian in London, they wont be on site bricks, if its early Victorian in the country, they will be. Even in London, bricks were made on site for a long time.

If theyre the later wire cut type, sure to be off site, but even at the end of Vic period, hand made stocks were standard.

NT