What is SBR? Uses, mixing ratios and SBR vs PVA
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SBR is a styrene butadiene rubber latex, a milky liquid you add to cement. It does three jobs: as a bonding agent it helps new mortar, render or screed grip an old surface; as an admixture in the mix it makes screeds, renders and mortars stronger, more flexible and water-resistant; and it works as a waterproofing slurry. Unlike PVA, SBR does not re-dissolve in water, so it is the one to use externally and in any damp or structural spot. Dilution depends on the job, roughly 1 part SBR to 3 or 4 parts water as a primer, or added to the gauging water as an admixture.
What is SBR?
SBR (styrene butadiene rubber) is a polymer latex you mix into cement-based products. Added to a mix or brushed on as a bonding coat, it improves adhesion, adds flexibility, and boosts water and chemical resistance and durability. It is suitable inside and out, including areas of continual or intermittent water contact, which is exactly where ordinary cement mixes and PVA fall down. Our Bondit SBR comes in a 5L tub.
What do you use SBR for?
Three main jobs, and it is genuinely multi-purpose:
- Bonding agent. Brushed on neat or as a slurry, it gives new render, screed or mortar a key to grip the old surface, no more new mix failing to stick to smooth or dense concrete.
- Admixture. Added to the gauging water, it makes screeds, renders and mortars stronger, less prone to shrinkage cracking, and water-resistant.
- Waterproofing slurry. Mixed with cement into a brushable slurry, it forms a water-resisting coat, useful on below-ground and damp masonry.
It is also a sound primer under tiling and levelling compounds on tricky backgrounds.
SBR as a bonding agent
This is the everyday use. Brush SBR (neat or diluted, per the tub) onto the sound, dust-free old surface, or mix a slurry bond coat of SBR, water and cement and brush that on. Then lay your render, screed or mortar while the bond coat is still tacky, wet-on-wet. Let the bond coat dry fully first and you lose most of the benefit, the whole point is that the new mix keys into it.
SBR as an admixture
Add SBR to the water you gauge the mix with. It makes a screed or render stronger, more flexible and water-resistant, which matters for thin screeds, patch repairs, external render and anywhere that takes water or foot traffic. It also improves adhesion of the mix in its own right. Follow the dose on the tub, it varies with the job.
What are the mixing ratios?
Ratios vary by manufacturer and job, so the tub is the final word, but as a guide:
| Use | Typical mix |
|---|---|
| Bonding primer | 1 part SBR to 3 to 4 parts water, brushed on |
| Slurry bond coat | SBR + water + cement to a brushable slurry |
| Admixture (screed, render, mortar) | SBR added to the gauging water, per the tub |
| Waterproofing slurry | SBR + cement, usually two coats |
Mix only what you can use before it goes off, and keep the surface and mix wet-on-wet where you are bonding.
SBR or PVA: which should you use?
They get used for the same bonding jobs, but they are not equal, and this is the one that costs people:
| SBR | PVA | |
|---|---|---|
| Water resistance | Water-resistant, does not re-dissolve | Water-soluble, re-emulsifies when wet |
| External use | Yes | No |
| Wet or damp areas | Yes | No |
| Admixture in a mix | Yes | No |
| Best for | External, structural, damp, admixture | Internal dry sealing and priming only |
The rule: PVA is fine for internal, dry priming and sealing; anywhere external, damp, or in the mix itself, use SBR. PVA re-dissolves when it gets wet, so a bond made with PVA outside or behind tiles in a wet area simply lets go. See the PVA bonding range for the internal jobs.
Common mistakes to avoid
- PVA outside or in a wet area. It re-dissolves and the bond fails. Use SBR anywhere damp or external.
- Letting the bond coat dry. Lay the mix wet-on-wet while the SBR coat is tacky, not after it has set.
- Wrong dilution. Too weak and it does little; follow the tub for the job.
- Bonding onto dust. The surface must be sound and clean or nothing holds.
The short version
SBR is a rubber latex you add to cement, as a bonding agent, a mix admixture and a waterproofing slurry. It is water-resistant, so it works externally and in damp and structural jobs where PVA (water-soluble, internal-dry only) would fail. Dilute to about 1:3 or 1:4 as a primer, or add to the mix water as an admixture, and always work bond coats wet-on-wet. Grab a tub from the SBR range.
By Steve, Builders Hardware Direct. Steve runs the BHD trade counter and has spent years supplying SBR, cement and building chemicals to builders and renderers across South Wales. Not sure how to use it? Call the counter on 01269 267030 or order online for fast UK delivery.
