A Question Of Rendering.

[derekpara]

Is the use of a good quality bonding agent an adequate alternative to 'keying' an exterior wall before re-rending and painting ?

Cheers.

d

[Zacs-Master]

I would imagine it would depend on the material being rendered. Smooth brick, probably not. Textured stone or brick, probably OK.

[derekpara]

Blimey, that was quick, ZM !

The existing surface is Weathershield painted rendering (slightly textured)

[Zacs-Master]

I'm pretty sure you would be mad to render (or attempt to) on top of paint.

[derekpara]

The builder has specified a bonding agent followed by a keyed cement undercoat then textured rendering followed by coloured Weathershield or similar.

[Zacs-Master]

Your builder must be assuming that the paint is keyed to the wall. Dangerous.

[KARL]

The durability of what is being proposed must be considered unknown. Bonding to a surface (whether glued on, keyed or whatever) will only prove as successful as the integrity of the material being bonded onto. In your case you might succeed to thoroughly bond to the paint but if the paint subsequently comes off then so will everything stuck to it. Should this happen then you will see the paint on the underside of everything that was stuck to it (inspecting the back is a standard way to establish what has failed). This is all about how faithfully you trust the paint to stay stuck - if sure then everything stuck to it will also stay. My worry would be about how the paint will react over time, with other materials applied to it.

[carrust]

Does the builder mean a tyrroleon(sp) coating or a sand & cement render? As Zacs has said, to put a sand & cement render on top of paint is foolhardy.

[derekpara]

The existing paint was only applied a year ago and is in excellent condition and firmly attached to a stable render. (The insurance current work is as a result of a botched surface wiring job by a communications company )

[Zacs-Master]

Applying a render over (which I presume you're going to paint) could cause condensation which could cause the render to blow in time.

[derekpara]

Well, Zac, it's my son's house and he intends to move in a year or two so maybe it will be someone else's problem !

[KARL]

I really cannot agree with the condensation theory - evoking the UK-staple bogeyman is completely unjustified in this case. However, the presence of the paint in the sandwich would always worry me. Ask those advocating the work method for a bond-backed 20 year guarantee (in case they disappear from existence) and watch them balk. This might work fine, but then again it might not - at least in patches sooner or later.

[The Builder]

Derek, inside the house, a keying agent is fine. That's how we plaster direct over tiles if we need to.
Exterior........ it's risky. The paint may well be stable. The trouble is, the bonding agent will ensure the render sticks to the paint - no problem.
Then .......... the paint starts to leave the masonry, and the whole lot is on the floor.

The simplest form of keying the surface is to just attack the paint with an angle grinder with a normal diamond disc.
Plenty of cuts with the grinder, and you at least have some mechanical key.

Then .......... PVA bonding agent, and off you go.

[derekpara]

Thanks, TB. That's the way we'll go.

D