How to Polish a Slate Floor
Of all the natural stones used for flooring, slate, with its rich coloration and texture of stunning rusty red, brown, and gray hues, is one of the most popular. It easily complements different construction styles in both modern and rustic home designs.
One of the slate’s greatest assets is the ability to get it in a variety of finishes, from honed, brushed, and even in a permanent wet look which can bring out the best of its natural beauty.
1. Restoring Slate and Other Natural Stones
The restoration of floors made of natural stones is a process of bringing the stone back to its natural state with all of its initial beauty and color, and depth. Polishing and buffing are two terms often used interchangeably. Polishing, however, refers to a more aggressive approach to clear away bumps, chips, and scratches.
At commercial and industrial levels, both methods are used to get rid of chips and pits, and they both require their own specialized types of equipment and tooling as well as their own specific techniques for doing so.
A regular weekly scheduled care and maintenance routine of sweeping and washing your floor will minimize the frequency of more elaborate polishing, burnishing, or even honing the stones.
Deep scratches or stained surfaces, either from neglecting to clean the dirt from footwear or from spills of oil, food, or even water on the floor, can all result in eventual severe damage and blotching.
The most serious damages to your floor could very well require the services of professionals with their inventory of specialized tools and equipment to restore damaged or worn-out stone back to its original natural state.
It should be mentioned, however, that any stone floors, no matter how carefully it has been looked after, will eventually show wear and tear, especially in busier areas where it’s submitted to heavy traffic from constant and regular usage.
2. Types of Damages Found on Natural Stone
It is perfectly normal for natural stone to eventually become damaged from everyday wear and tear. This is especially true on certain areas of your floor submitted to a lot of foot traffic or where furniture is frequently moved around. The same damages can happen to inside flooring as well as outside—the restoration, however, will be the same.
2.1 - Stains - More common on interior flooring, food stains, as well as acidic cleansers, will stain natural stone. Oil, rust, and water stains can leave you with discolorations very difficult or impossible to clean off. Water stains don’t result from the water itself but from whatever minerals or other deposits are mixed in with the water.
The most common rust stains are usually the result of nails, screws, or bolts left dragging outside. They can also result from tin cans, metal furniture legs, or any other metallic object left sitting on the tiles.
2.2 - Scratch Marks - This type of damage is more serious on high-traffic areas of your floor, such as pathways leading to doorways or in front of furniture like sofas, or around a dining room table, etc.
The scratches are caused by the dirt, sand, and other abrasives harder than the stone gets rubbed against it. The stone around or under heavy furniture, for example, will likely be submitted to such wear when moved or drug across the stone paving.
2.3 - Etching - This action occurs when the top layers of the stone surface are dissolved or eaten away from use wear or from the spill of an acidic chemical such as vinegar.
Etching will leave a dull spot on the stone surface, resembling a clear watermark—the longer the exposure, however, the more damage will be submitted to the stone, and the more layers will be affected. Polishing and burnishing the stone is the only true solution to resolve that issue.
2.4 - Stun Marks - A stun mark will result from a hard object impacting the stone surface. Such hit marks leave a surface-deep white discoloration, but they can also damage the stone weakening the link between the layers.
Furthermore, depending on the weight of the impact, the slab of stone could be fractured even through its full thickness if it hits hard enough.
2.5 - Ice Damage - Seen only on exterior paving, untreated stone can also suffer from ice damage. Given the fact that the porosity of slate is 0.88% whereas the porosity of sandstone is 20%, different stones will absorb water at different rates.
Depending on how much water from melting snow or otherwise is soaked up by the stone, it will definitely freeze up once the temperature drops below freezing.
Since freezing water expands with tremendous force, depending on the porosity and water level absorption of the stone, the formation of ice will chip or crack the stone from whichever fissure it accumulated.
2.6 - Hazing - Rain and groundwater is a mixed electrolyte that contains varying amounts of sodium, potassium, magnesium, calcium, chloride, bicarbonate, and sulfate, to name just a few of its by-products.
The hazy chalky substance that ends up on the surface of exterior stone flooring is often the result of rain or groundwater that found its way onto the flooring, leaving the salts drying on the surface of the stone once the moisture has evaporated from it. It is a safe natural process that can be easily cleaned up.
2.7 - Poor Installation - Natural stones that were poorly installed can eventually end up cracking due to lower grade pavers, a paving base poorly executed and not providing a uniform flat surface, nearby trees extending their roots underneath the paving, or poor jointing.
3. The Restoration Process
Following the various causes of damages listed above, you probably came to realize that some restoration projects will be labor-intensive to bring the stone back to its pristine condition.
In some cases, you’ll possibly have to resort to professional help. Some of the restoration and repair procedures required to restore the stone to its natural luster, such as grinding, honing, and polishing, can only be done with specialized equipment.
3.1 - Burnishing Versus Polishing - The burnishing process or stone grinding is similar to the polishing of natural stone. It differs from polishing in the way it uses several diamond burnishing pads in a specific sequence.
Since the abrasive grit will generate a substantial amount of heat, lubrication of the surfaces can be achieved using small amounts of water. The burnishing machine works at a faster pace than a buffer machine.
With each pad at different grit sizes, the burnishing process is started with the largest size grit first—a smaller grit number depicts larger size particles— gradually changing it for finer grits, each one gradually cutting down through the stone surface with the finest grit bringing the finish to a finely polished surface.
The process can be started using a 400 grit pad, and then replacing it with an 800 grit, and gradually replacing it to end up finishing the process with a 1500 grit super fine finish for the same result as you would get from polishing.
So burnishing is a type of polishing. Burnishing, however, needs to be done only when the stone has been submitted to extreme damage or etching.
3.2 - Honing - In this phase of the restoration process, a fine abrasive powder is mixed with the water while passing the disk pad over the damaged area.
3.3 - Polishing - Polishing is the process of making a surface smooth and shiny by rubbing it with a polishing compound or waxing it with a chemical treatment leaving you with a clean surface. As with the burnishing process, polishing with abrasives starts with a coarser grain abrasive and gradually progresses to finer grits.
At this stage of the restoring process, polishing is where the stone’s original shine, luster, and color are finally restored to the stone surface.
This step of the polishing process is finally followed by the application of an impregnator or a sealer to add significantly increased resistance to the floor surface and avoid further premature staining and minor scratching of the stone.
The application of sealant should be repeated regularly, the frequency of which depends on whether the building it's in is residential or commercial.
3.4 - Penetrating Sealers - Although slate is often left to its natural dull and mat surface, you might prefer to brighten up its color by adding a shine to the tile flooring with an enhancing sealer.
Sealers for natural stone are available as water-based as well as solvent-based enhancers, the latter being recommended for their ease of application and their ability to get readily soaked into the stone.
On a new floor, a solvent-based sealer should be applied about 72 hours after the installation, before any spill or dirt contaminate the stone surface.
Older installations that already suffered all different sorts of spills and other damages will obviously necessitate a thorough cleaning and stripping of any previous types of coatings with possibly some degree of polishing to bring the slate to its original look.
The application of the enhancing sealer at this point will add substantial protection to the restored surface along with the desired shine, depending on the type of sealer used.
If you’re looking for enriched colors and added sheen to brighten up the stone, the application of a carefully chosen sealer is the way to enhance the stone’s natural beauty.
A solvent-based enhancing sealer is always recommended for adding shine to the tile flooring primarily because of its ability to soak into the stone but also for its ease of application compared to a water-based sealer.
The downside, of course, is from the volatile organic compounds and the smell emitted from the solvents that will spread throughout the room and even the whole house and subside for several hours.
For a more natural look of slate, a clear impregnating sealer would provide a better-suited finish that would maintain the original texture and appearance of the natural stone.
You should also note that the water-based impregnating sealer is better suited for this as it “disappears” once it’s soaked into the stone. You should note that the stone will appear darker until completely dried out.
Warning! - Make sure before starting the application of the sealer that the room is well-ventilated and use a respirator for added protection. Also, make sure that you test your sealer on an inconspicuous tile to check the outcome before proceeding to coat the whole floor.
The enhancing sealer can be applied by simply pouring it directly on the stone and spreading it evenly with a towel over the tiles. You can then let it sit for about 10 minutes and add more coats if desired or as needed. Both the enhancing and the impregnating clear sealer are applied in the same way.
3.5 - Topical Sealers - If you’re looking for a glossy polished look for your slate floor, you should consider using a topical glossy sealer. Again available as a solvent-based and as a water-based sealer, the solvent-based has proven to be much more durable than its counterpart.
The sealer’s coating doesn’t soak into the stone but builds up on the surface and, therefore, will not adhere to a polished surface.
As a surface coating sealer, they’re better at protecting against spills and stains, but on the downside, they’ll tend to emphasize any issues such as scuff marks and scratches, especially where there’s more foot traffic, as they might show more on a glistening surface.
For that reason, topical glossy sealers require more care and recoating, requiring stripping the floor prior to a new application to avoid a sealer build-up.
Such build-up will eventually give your floor the appearance of a waxy or plastic look while encouraging dirt buildup and more frequent cleaning and buffing, and at a certain point, it will need to be stripped.
You should therefore find out about your preferred sealer, as some topical sealers can be quite easily stripped, while others made from polyurethane or epoxy polymers can be much more resilient and difficult to remove.
The application of a topical sealer which is slightly different than the penetrating sealer, should be done in smaller sections of the floor, progressing throughout one section at a time.
As each section is glazed with a light coat of glossy sealer, you then have to avoid touching the sealer for at least 4 hours while it’s drying as you finish coating the floor surface.
4. Buffing the Surface
A floor buffer is a machine quite similar to a floor burnisher. The burnisher rotates the pad at speeds between 1000 and 3000 RPM. to minimize the number of passes over the stone in order to obtain a nice smooth and shiny surface.
The floor buffer, also referred to as a floor machine, a floor polisher, a side-to-side machine, a swing machine, or a bonnet cleaner, can be used for several different operations.
They can be used to clean, scrub, remove scuff marks and dirt, strip the floor, and buff the floor to restore its original shine. The buffer’s pad turns at a speed between 150 and 330 RPMs.
Buffing or spray buffing requires a floor buffer equipped with a special buffing pad that is passed over the floor as it is being sprayed from a spray bottle with a special solution designed to fill in nicks, scratches, and smudges to bring the surface to a nice shine.
For more information on more maintenance tips and other slate projects, check out our pieces on “5 Slate Floor Maintenance Tips”, “Slate Tile Flooring vs Ceramic Tile Flooring”, and “A List of New Slate Tile Projects to Take On.”