Making smooth sandstone comes down to two paths: smoothing natural sandstone you already have, or casting your own synthetic sandstone with a smooth finish from the start. Both are straightforward DIY projects, but each requires different techniques and materials. The approach you choose depends on whether you’re working with an existing piece of stone or building one from scratch.
Smoothing Natural Sandstone
Sandstone is a sedimentary rock, softer than granite or marble, which makes it easier to shape but also easier to damage if you rush. The key to a smooth finish is working through progressively finer grits of sandpaper, never skipping more than one step in the sequence.
Start with 60 to 80 grit sandpaper to remove major bumps, rough spots, or saw marks. This coarse stage does the heavy lifting. From there, move to 180 or 220 grit to smooth out the scratches left by the first pass. Then progress through 320, 600, and 1200 grit. Each stage erases the scratch pattern from the one before it. If you want a polished, almost glassy surface, continue to 3000 grit and beyond, up to 8000 or even 14000 for a mirror-like finish.
One important rule: don’t jump grits. Going from 80 straight to 600, for example, means the fine paper can’t remove the deep scratches left by the coarse paper. You’ll end up with a surface that looks smooth from a distance but feels rough to the touch. Each grit step should be roughly double the previous number (80 to 180, 180 to 320, and so on).
Wet sanding works better than dry sanding for sandstone. Running water over the surface while you sand reduces dust, keeps the paper from clogging, and produces a more even finish. It also dramatically cuts down on airborne silica dust, which is a serious health concern covered below.
Using a Rock Tumbler for Smaller Pieces
If you’re smoothing small sandstone pieces, a rotary rock tumbler does the work for you. The process takes about four weeks total, broken into four one-week stages. Load the barrel half to two-thirds full with stones of similar hardness. Sandstone is soft compared to agate or jasper, so tumble it only with other sandstone or similarly soft rocks. Mixing hard and soft stones together grinds the soft ones down to nothing before the hard ones are even shaped.
For each pound of rock, add two level tablespoons of grit at each stage. The four stages use increasingly fine grit: 60/90 silicon carbide for coarse grinding (often run for two weeks instead of one for better shaping), 150/220 for medium grinding, 500 or 600 for fine grinding, and aluminum oxide polish for the final week. Clean the barrel, lid, and stones thoroughly between each stage. Even a few stray coarse grit particles left behind will scratch your stones during the finer stages and ruin days of work.
Casting Smooth Sandstone From Scratch
You can make convincing artificial sandstone by mixing sand with a binding agent and casting it in molds. The result looks and feels like natural stone but gives you complete control over shape, color, and surface texture.
Choosing Your Binder
The simplest binder is Portland cement. A typical mix uses about 3 to 5 percent cement by volume relative to sand. For a stronger bond, use one part pure sand mixed with two parts sand that has been pre-coated with a thin layer of cement or gel. This mimics the way natural sandstone forms, where mineral cement fills the tiny gaps between sand grains. Sodium silicate (often called water glass) is another option that produces a result chemically closer to natural sandstone, since real sandstone is often cemented by silica deposited between grains over millions of years.
Getting a Bubble-Free Surface
The biggest challenge when casting is air bubbles. They leave tiny pits in the surface that make your stone feel rough instead of smooth. Several techniques help eliminate them:
- Vibrating table: Place your filled mold on a vibrating table immediately after pouring. This shakes bubbles to the surface where they pop. A vibrating table reduces bubbles dramatically compared to any other method.
- Pounding: If you don’t have a vibrating table, firmly pound the work surface with your hand or a mallet while filling the mold. This forces trapped air upward.
- Blowing: After vibrating or pounding, blow across the top of the mold to pop any bubbles sitting on the surface.
- Manual removal: Check the mold’s detailed areas for stubborn bubbles and push them out with your finger, a cotton swab, or a dull pencil tip.
Spray the mold lightly with a release agent before pouring. This prevents the casting material from sticking and also helps air escape from corners and fine details. Pour slowly and steadily rather than dumping the mix in all at once.
Hardening Soft or Crumbly Sandstone
Some sandstone is too crumbly or “friable” to sand directly. The grains just pull loose instead of smoothing down. In these cases, you need to consolidate the stone first by soaking it with a liquid that penetrates the pores and hardens inside.
For sandstone specifically, consolidants that form silica inside the stone are the best match, since natural sandstone is already silica-based. Products in the alkoxysilane family are widely considered the most effective option by stone conservators. They penetrate deep into the stone and bond chemically with existing sand grains, strengthening the entire piece rather than just coating the surface. Acrylic-based consolidants can also work, hardening the surface effectively as long as they penetrate deeply enough and cure completely.
Wax is a simpler, more accessible option for small projects. Melted wax soaks into the pores, solidifies, and holds loose grains in place. It won’t make the stone as hard as a chemical consolidant, but it’s easy to apply and gives the surface a smoother feel immediately.
Sealing for a Smooth Finish
Even after sanding, sandstone remains porous. A sealant fills those microscopic pores and creates a surface that feels genuinely smooth to the touch. You have two main choices depending on the look you want.
Penetrating sealers soak into the stone without changing its appearance much. They protect against moisture and staining but don’t add significant gloss. Topical sealers sit on the surface and create a visible coating. A “wet look” topical sealer deepens the stone’s natural color and adds a slight sheen that makes the surface feel slick and polished. For the smoothest possible result, apply a topical sealer after completing your full sanding progression. The sealer fills any remaining micro-pits that sanding alone couldn’t reach.
Apply sealant in thin, even coats. Two or three light coats outperform one thick coat, which can dry unevenly or trap cloudiness. Let each coat dry fully before adding the next.
Silica Dust and Safety
Sandstone is mostly quartz, and grinding or sanding it releases crystalline silica dust. This is not ordinary dust. Inhaling fine silica particles causes permanent lung damage over time. OSHA sets the workplace exposure limit at just 50 micrograms per cubic meter of air, averaged over eight hours. To put that in perspective, that amount is invisible to the naked eye.
Whenever you’re cutting, grinding, or sanding sandstone, wear a respirator rated for fine particulates (N95 at minimum, P100 preferred). Safety glasses and work gloves round out the basics. Wet methods are strongly preferred over dry methods because water binds the dust before it becomes airborne. If you’re working indoors, do it in a well-ventilated space or near a dust extraction system. Working outdoors with a breeze at your back is the simplest way to keep exposure low.
Rock tumblers are sealed during operation, so they produce virtually no airborne dust during the tumbling stages. The main exposure risk comes when you open the barrel to clean and inspect stones between grit stages. Do this over a sink with running water rather than dumping the slurry where it can dry and become airborne later.