Mastering how to grind and polish concrete countertops is critical to any project’s successful completion.
Concrete is a mixture of cement, rock aggregate, sand, and water. Cement by itself is very brittle, but when rocks and sand are added, a matrix is formed that strengthens the mix. The cement is like the glue, and it fills in the small space between the aggregate and sand. This makes concrete much stronger than cement on it’s own. Because the rock aggregate in concrete mixes are quarried regionally, the types of rocks you see will be different depending on where you are in the world. The concrete pieces in this guide are made from a 5000 psi. concrete mix produced with Northern California aggregate.
Normally when casting concrete, the cement paste forms a uniform layer on the surfaces of the piece, producing a piece that is one solid color with minor variations. This is referred to as a cast-finish. The cement layer can be removed with very light polishing. This will begin to expose the fine sand in the mix, called a light-polish. Deeper polishing will expose the rock aggregate and produce a ground-finish. Very little material needs to be removed to expose the aggregate and achieve a ground finish (about 1/16″).
This guide will show you how to polish concrete countertops. There are a range of finishes possible from a single piece of concrete. Normally polishing is accomplished using a variable speed concrete polisher with a set of diamond sanding discs. The polisher is usually water fed and is attached to a normal garden spout. The water helps cool the pads down and also keeps dust to a minimum.
Polishing concrete countertops is not dangerous, but water and electricity do not mix well. If your polisher doesn’t have a Ground Fault Circuit Interrupter, be extremely careful to avoid electrocution. Even with a GFCI, it’s a good idea to wear rubber gloves to help insulate yourself.
Wear a particle mask if you’re doing any kind of dry grinding, concrete dust is caustic.
Step 2 – Cast Finish, Light Polish, Ground Finish
From a single piece of concrete, a range of finishes are available depending on if you polish, and how deep you polish.
Uniform Finish – The concrete has a uniform color, because when it’s being cast, there is a thin layer of cement paste that forms on the inside surfaces of the form work.
Light Polish - As soon as you begin polishing the surface, the thin layer of cement is removed, and the fine sand aggregate is exposed.
Ground Finish – As you polish deeper into the concrete, you begin to expose the larger aggregate in the concrete mix.
Step 3 – Grinding vs Polishing
Grinding can be thought of as the roughest way to flatten a concrete surface. Polishing is when you take that ground surface to a progressively smoother, shiny finish.
When doing pre-cast concrete projects, it’s not usually necessary to use a grinder. Since the formwork is doing most of the shaping, the only place you would need to use a grinder would be on a part of the project that is hidden (like the back side of a concrete countertop).
Step 4 – Variable Speed Polisher
A variable speed wet polisher is a versatile tool that can be used on any kind of decorative concrete work, and can also be used on glass or other stone work.
The Alpha polisher shown uses normal wall power (110V in USA) and should be plugged into a GFCI. The GFCI a breaker that will stop power to the tool if the risk of electrocution is present.
The water feed line is just like a normal gardening hose and threads onto a standard spout. There is a valve that controls the rate that the water is fed through the center of the arbor. In some polishers, the water feed line is actually cooling the bearings inside the tool, and running it dry can toast the bearings.
Step 5 – Types of Polishing Pads
Soft Pads - Require a rubber backer that screws onto the polisher. Soft pads are better for beginners because they’re more forgiving. They’re also able to polish contoured surfaces. Usually used with a water-fed concrete polisher.
Rigid Pads – Require a quick-release mechanism that attaches to the polisher. Rigid pads polish more quickly than soft pads, but they can also gouge the surface if the pad isn’t held perfectly flat. Usually used with a water-fed concrete polisher.
Wet / Dry Pads – Require a quick-release mechanism, but unlike other pads, these are safe to use on a high speed angle grinder.
Step 6 – Wet Polishing Preparation
Have a good idea of what you want to achieve before beginning the polishing process. It’s always possible to remove more material, but you can’t put it back, so work carefully.
Wear rubber gloves
Consider wearing a rubber apron and rubber boots.
Elevate the piece on scrap 2″ foam.
Polish outdoors, in the shade if possible, or indoors with a polishing table.
Build a polishing table (sloped table covered with a plastic tarp, 6″ high walls, water drains to a plastic bin, pump to recirculate water, plumbing for water hose valves).
Step 7 – Initial Polish
For a ground finish, start with the lowest grit pad you have, usually 60 grit. Polish the piece evenly, until a uniform amount of aggregate is exposed, then continue up to the next pad.
For a light polish, start with mid-range pad, like a 300. You can always remove more material, but you can’t go back, so be careful not to start too low if you don’t want a fully ground finish.
Tape off anywhere you don’t want to polish with a few layers of masking tape.
Step 8 – How to Polish
Turn on the water feed.
Lift the polisher above the surface and turn it on.
Lower the polisher onto the concrete and let the weight of the tool do the work. Keep the pad as flat as possible.
Move around the piece evenly, in circular motions. Don’t start or stop in one place for too long.
If you have a large area to polish, focus on small sections rather than trying to do too much at once.
Progress to the next pad when the surface is uniformly polished.
Step 9 – Fill Holes with Slurry
Halfway through the polishing piece (around 400-grit), fill the exposed holes with slurry (cement paste, pigment, and water).
Resume polishing after the slurry has dried for a day.
Read the How-To Slurry Guide for more information.
Step 10 – Final Polish
Remove the slurry by polishing the piece using the higher grit pads (400-grit and up).
The final step is to seal the piece. This will help keep it looking nice and preserve the polished finish. Check out the How-To Seal Guide for more information.