If you've spent any time looking at old masonry, you've probably realized that pointing sandstone walls isn't quite as straightforward as working with modern brick. Sandstone has a personality—it's porous, often quite soft, and it reacts to the weather in ways that can be a bit frustrating if you don't treat it right. If you use the wrong materials or a sloppy technique, you aren't just making the wall look messy; you might actually be causing long-term damage to the stone itself.
Why Sandstone Needs Special Care
The most important thing to understand before you even touch a trowel is that sandstone needs to breathe. It's a natural sponge. When it rains, the stone absorbs moisture, and when the sun comes out, that moisture needs a way to escape. In a perfectly functioning wall, the mortar joints act like a wick, drawing the water out so it can evaporate.
The biggest mistake people make when pointing sandstone walls is using a modern cement-based mortar. Cement is incredibly hard and almost entirely waterproof. If you pack a sandstone wall with cement, the moisture gets trapped inside the stone. When winter hits and that trapped water freezes, it expands and blows the face off your beautiful sandstone. This is called spalling, and once it starts, there's no real way to "fix" the stone other than replacing the block. That's why we almost always talk about lime mortar when we're dealing with this kind of work.
Getting the Old Stuff Out
Before you can put anything new in, you've got to get the crumbling, sandy mess out of the joints. This is easily the most tedious part of the job, but you can't skip it. You're looking to rake out the old mortar to a depth of about twice the width of the joint. So, if your gap is 10mm wide, you want to go back about 20mm.
Try to avoid using power tools if you can help it. I know it's tempting to grab an angle grinder and zip through the lines, but it's way too easy to slip and slice into the stone. Once you've nicked the edge of a sandstone block, that mark is there forever. A hammer and a narrow "plupping" chisel (or even an old screwdriver for the tight spots) are usually your best bets. It's slower, sure, but it keeps the character of the stone intact.
Once you've raked everything out, give the wall a good brush down and a light spray with water. You don't want the wall soaking wet, but if the stone is bone-dry, it'll suck the moisture right out of your new mortar before it has a chance to bond.
Mixing the Right Mortar
For most pointing sandstone walls, you'll want to use Natural Hydraulic Lime (NHL). Usually, NHL 3.5 is the middle-of-the-road choice that works for most exterior walls. You mix this with sand—usually a ratio of one part lime to two or three parts sand.
The sand you choose is actually what dictates the look of the wall. If you use fine silver sand, it'll look smooth and clinical. If you use a coarse, "sharp" sand with little pebbles in it, you'll get a much more traditional, rugged finish that blends into the stone. I always recommend doing a few tiny test patches in an inconspicuous corner. Mortar looks totally different when it's wet versus when it's fully cured, so give it a few days to dry before you decide if the color is right.
Putting the New Mortar In
Now for the actual pointing. You don't want to just smear the mortar over the face of the stone. The goal is to pack it tightly into the back of the joint first. If there are big voids or "pockets" behind the surface, fill those first with small bits of stone (called pinnings) and mortar before you do the final face.
Use a pointing trowel that's slightly narrower than the joint itself. It feels a bit like icing a cake, but with more physical effort. You want to push the mortar in firmly to make sure there are no air gaps. Don't worry too much if it looks a bit "proud" or messy at this stage; the magic happens in the finishing.
The Secret is in the Timing
One of the most common errors is trying to finish the joint as soon as the mortar is in. If you try to brush it or strike it while it's still wet and "fatty," you'll just smear lime all over the face of the sandstone, and that's a nightmare to clean off later.
You have to wait for the mortar to "green up." This means it's started to set and feels like firm clay. If you press your thumb into it, you shouldn't leave a deep print, but it shouldn't be rock hard either. Depending on the weather, this could take two hours or six.
Once it's at that stage, take a stiff churn brush (a brush with short, stiff bristles) and beat the mortar back. Don't brush it side-to-side like you're sweeping a floor; literally tap or "stipple" it with the ends of the bristles. This compacts the mortar further and brings the aggregate (the sand) to the surface. It gives the joint a beautiful, weathered texture that looks like it's been there for a hundred years.
Dealing with the Weather
Since we're usually using lime for pointing sandstone walls, you've got to be a bit of a meteorologist. Lime mortar is picky. It hates direct, scorching sun because it dries out too fast and cracks. It absolutely loathes frost. If the water in the mortar freezes before it's cured, the whole lot will just turn to dust and fall out in the spring.
If it's a hot day, you might need to hang some damp burlap (hessian) over the wall to keep it cool and moist. If there's a risk of frost, you'll need to wrap the wall up warm. It sounds like a lot of babying, but considering how long a good pointing job is supposed to last—fifty years or more—it's worth the extra effort for a week.
A Few Things to Avoid
While we're on the subject, let's talk about what not to do. First, avoid "over-pointing." This is when the mortar is spread over the edges of the stone to hide chips. It might look okay for a month, but because that thin layer of mortar has no depth, it'll quickly crack and peel off, leaving the wall looking worse than when you started. Always keep the mortar slightly recessed or flush with the stone's edge.
Also, don't use a "tuck pointing" style with bright white lines unless you're working on a very specific type of period property that calls for it. On most sandstone walls, you want the mortar to be a supporting character, not the star of the show. The stone should be the first thing you see.
Final Thoughts
At the end of the day, pointing sandstone walls is more of a craft than a chore. It takes a bit of patience to get the mix right and a bit of "feel" to know exactly when the mortar is ready to be finished. But there's something incredibly satisfying about seeing a crumbling, damp-looking wall transformed into something solid and sharp.
Take your time with the prep work, ditch the cement for some good hydraulic lime, and wait for that perfect "green" moment to finish the joints. Your sandstone will thank you by staying solid for several more decades, and it'll look fantastic while doing it.