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I am a plasterer by trade and we have an exterior rendering job coming.

The exterior face of the house is red brick and has to be scratched, then dry dashed over it. I have heard some horror stories about the scratch not bonding to the brick and I was just wondering if anyone has any help or advice on what mix, additive, to use?

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In the UK, in situations where you might be slightly concerned about the surface being rendered, a coat of equal parts water & UniBond PVA Adhesive & Sealer is applied to the surface prior to the scratch-coat going on.

If the brickwork is quite porous, wetting it down beforehand (just prior to applying the UniBond PVA Adhesive & Sealer) with a hose wouldn't be the worst idea, also wetting down the scratch coat before the top coat is applied isn't a bad idea (especially if the weather is on the warm side).

Adding a plasticizer and waterproofing agent to the scratch coat is worthwhile (normally specified, required).

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