Sealing Cast Concrete Furniture

Concrete looks solid and behaves like a sponge. The cement paste between the aggregate is full of pores, and left alone they drink whatever lands on them. That is why every piece of concrete furniture worth owning is sealed, and why the sealer is the part you have to think about rather than the concrete.

What a sealer is doing

Two families, two jobs.

• Penetrating sealers soak into the pores and line them, so liquid beads up instead of wicking in. They barely change how the surface looks or feels. They wear from the top down and you top them up.
• Film sealers sit on top as a thin layer of plastic. They give more resistance to staining and usually add some sheen. They scratch, and a deep scratch means liquid is under the film where you cannot wipe it.

A matte sealer aims for the first effect with a little of the second: protection without turning raw cast concrete into something that looks laminated. That is the whole point of casting a piece rather than buying a worktop.

Stains, etches and the difference

People call everything a stain. Two different things are happening.

A stain is a liquid sitting in the pores and colouring them. Coffee, red wine, oil, ink. If you catch it early you lift most of it, and a sealed surface gives you long enough to do that.

An etch is chemical damage. Acids dissolve cement paste. Lemon juice, vinegar, descaler, some wines and some cleaning sprays will leave a slightly rougher, duller patch, and that patch is not a mark on the surface, it is a shallow hole in it. Cleaning does nothing because there is nothing left to clean.

This is why the rule for concrete is so blunt: nothing acidic, ever, not even once as an experiment.

The water test

You do not need to guess when a piece needs resealing. Put three or four drops of water on the areas that get used, wait five minutes and look.

• Drops sit proud, concrete under them unchanged: sealed.
• Drops flatten and the concrete darkens slightly: the sealer is thin here and the piece is ready for a refresh.
• Water vanishes into the surface in under a minute: unsealed or worn through.

Test more than one spot. A bench wears where people sit and where their heels land, not evenly across the slab.

Living with it day to day

• Wipe spills when they happen rather than when you next walk past.
• Warm water and a drop of washing-up liquid, then dry.
• Use a mat or a board under anything oily. Oil is the one that moves slowly and shows up a week later.
• Keep acidic cleaners in a different cupboard, so nobody helpful reaches for one.
• Do not drag ceramics across the surface. Unglazed foot rings on pots are abrasive.

Outdoors

Sealed concrete handles weather better than almost anything else in a workshop, which is why a concrete bench works on a terrace where a wool sofa would be ruined in a weekend. Freeze and thaw is the thing to respect: water that sits in an unsealed pore and then freezes expands and chips the surface from inside.

So for an outdoor piece, keep the sealer current, do not let the base sit in a puddle all winter, and check the underside and edges, which are the parts nobody looks at and the parts that stay wet longest.

Where a piece wears first

Sealer does not wear evenly, so checking a whole slab tells you less than checking the four places that take the use.

• The front edge of a seat. Where people slide on and off, and where trouser rivets and belt buckles land.
• The landing zone. On a table, the arc in front of each chair. On a bench, the strip where heels rest.
• Around anything that lives there. A plant pot, a lamp base, a bowl that never moves. These trap moisture underneath and the sealer under them ages differently from the sealer around them.
• Corners and arrises. The sharp edge of a cast slab gets knocked, and the sealer there is thinnest to begin with.

Run the water test on those four spots rather than in the middle, and you will catch a thinning sealer a year before it becomes a stain.

Marks you should not fight

Cast concrete is not a printed surface. Small voids, a faint swirl from the pour, a slight difference in tone from one end of a slab to the other: these come out of the mould and they are the reason the material reads as real. Trying to scrub them out with an abrasive just burnishes the cement and leaves a shiny patch that is far more obvious than the mark was.

The honest division is simple. Marks from the pour are the material. Marks from a lemon are your fault. Marks from a sealer that is three years old are maintenance, and maintenance is twenty minutes with a cloth.