9 Step Sandstone Patio Pointing with Marshalls Slurry Over a 25-Square-Metre Area

Start with the joint width, because the product fails outside its tolerance

Marshalls Slurry Grout is rated for joints between 5mm and 50mm wide and a minimum depth of around 20mm. Indian sandstone laid by hand rarely arrives at a tidy 10mm gap. Calibrated slabs at 22mm thickness give you a usable depth once the bedding mortar sits 5mm proud of the sub-base, but riven sandstone with thickness variation across a single slab will leave you with shallow pockets that the slurry slumps out of before it cures. Walk the whole 25 square metres and measure the narrowest joints with a tape. Anything below 5mm gets raked wider with a 4mm diamond blade on an angle grinder before you mix a single tub.

The slurry method works because the grout is wet enough to self-level into the joint when you squeegee it across the surface. That same fluidity means a joint shallower than 20mm holds too little material to bond, and it cracks out within a frost season. The product datasheet pegs coverage at 25 square metres per two tubs at 10mm wide by 20mm deep. Wider joints, common on riven stone, pull that down to three tubs fast.

Wet the slabs first, and time it against the weather

Dampen the entire surface before the grout touches it. Dry sandstone is porous and pulls water out of the slurry on contact, which leaves a dusty, weak surface skin and stains the slab faces a darker patch where the grout grabbed. A garden hose on a fine spray over the full 25 square metres, then let surface water drain off until the slabs are damp but not pooling. Standing water in the joints dilutes the mix and ruins the colour consistency between batches.

Air and surface temperature drive everything from here. Marshalls specifies an application range of 5C to 25C. Below 5C the cement chemistry stalls and the joint stays soft for days. Above 25C, or in direct sun on dark Kandla Grey, the surface flashes off before you finish the wash, and dried slurry residue bonds to the slab face where no scrubbing shifts it cleanly. Overcast and 12C to 18C is the working window most installers wait for. If the forecast shows rain inside six hours of your pour, stop. Rain on uncured slurry washes pigment out of the joints in streaks.

Mix to a pourable consistency, one tub at a time

Empty one 20kg tub into a tub trug or a forced-action mixer and add clean water in stages. The target is a thick, pourable cream that holds a soft ridge for a second when you draw the paddle through it. Too dry and it will not flow into the joints under the squeegee. Too wet and the pigment separates, leaving pale, weak joints that powder under a thumbnail once cured.

Mix only what you can place and wash off inside about 30 minutes. The working time shortens as the temperature climbs. Keep the water ratio identical across every batch by measuring it, not eyeballing it. A litre jug and a written count per tub keeps the joint colour even across the whole 25 square metres. Inconsistent water means a patio that dries to a patchwork of light and dark joints, and there is no fixing it after cure short of grinding the joints out.

Pour, squeegee, and pack the joints in one pass

Tip the mixed slurry onto the damp slabs in a manageable ribbon, working an area of two to three square metres at a time. Use a rubber-bladed squeegee to drag the grout diagonally across the joints from multiple angles. The diagonal stroke forces material down into the gap instead of skating it over the top. Work it back and forth until the joints are visibly full and slightly proud.

Then consolidate. A pointing bar or a striking tool run along each joint compacts the slurry and drives out trapped air, which is where pinholes and surface cracks start. On a 25 square metre patio with standard 600x600 calibrated slabs, you are looking at roughly 80 to 90 linear metres of joint to strike. This is the slow part. Skipping it leaves voids that fill with water, freeze, and pop the joint surface within a winter.

Keep a second person feeding mixed batches while the first squeegees and strikes. Working solo over 25 square metres means batches cure in the trug before you reach them.

The wash-off window is the whole job

This is where slurry pointing is won or lost. Once a section is poured, squeegeed, and struck, the grout begins to go off on the slab faces. You have to wash the residue off the stone before it hardens, but not so early that you wash it out of the joints.

The sign to start is when the surface slurry dulls from wet-shiny to a matte film, typically 15 to 40 minutes depending on temperature. Use a clean sponge float and clean water, wrung out so it is damp not dripping. Drag it diagonally across the slabs, never along the joint line, to lift the film without scooping grout out of the gap. Rinse the float in a separate bucket every few strokes and change the water often. Grey water smeared back across the stone is the most common cause of a hazy, milky finish that owners notice the moment the patio dries.

A second, lighter wash follows the first once the joints have firmed. On textured riven sandstone the pitting holds residue that a single pass misses, so a soft brush works the film out of the surface texture. Kandla Grey and Raj Green sandstone show every missed smear once dry. The film does not always appear until the slab dries fully, hours later, by which point it has bonded.

If a haze does set, a proprietary patio cement residue remover based on a buffered acid will lift it, but it can also dull the natural colour of the stone and needs a thorough rinse afterward. Avoiding the haze in the first place beats chasing it.

What the slurry method costs you in flexibility

Slurry grout sets hard and rigid. It bonds across the joint and locks the slabs into a monolithic surface, which is fine on a properly bedded, fully mortared base over a compacted sub-base. On a patio laid on spot dabs or a weak bed, that rigidity is a liability, because any slab movement cracks the joint rather than flexing with it. A flexible jointing compound forgives minor movement; slurry does not. Check the bedding is full and sound before committing to a rigid grout.

Leave it alone while it cures

Keep foot traffic off the patio for at least 24 hours, and garden furniture or planters off for a few days. The joint surface stays vulnerable to scuffing long after it feels walkable.

The numbers that decide your tub count

Work the coverage before you buy. At a 10mm joint width and 20mm depth across 600x600 slabs, each square metre carries roughly 3.3 linear metres of joint. Over 25 square metres that is about 83 linear metres. Marshalls rates two 20kg tubs for that benchmark, but two factors push it higher: wider joints on riven stone, and the volume lost to striking and wash-off waste. A joint averaging 15mm wide instead of 10mm needs around 50 percent more material by volume. Buy three tubs for a riven 25 square metre job and keep the receipt on the unopened one.

The per-tub yield also drops if your joint depth runs deeper than 20mm, which happens wherever the bedding sits low under a thick slab. Two installers grouting identical 25 square metre patios can finish one with two tubs and the other needing four, purely on joint geometry. Measure, do not assume the box figure applies to your stone.

What the datasheet cannot tell you is how your specific batch of sandstone will hold pigment in its surface pores, and whether the colour you wash onto Raj Green at 14C will dry to the same tone as the section you finished when the cloud broke and the temperature climbed.