Using an angle grinder on a concrete low spot

How Do You Fix Low Spots in Concrete?

Using an angle grinder on a concrete low spotFix concrete low spotsLow spots are uneven dips in a flat concrete floor. They are caused by installation mis-steps. For your safety and to stop the damage, you should fix garage floor low spots.

How do you fix a garage floor low spot? Use a de-greaser to remove oil from the surface. Use a wire brush, or a grinder or an acid-substitute, to prepare the concrete. For large areas use a floor buffer with a grinding attachment. Vacuum up loose material. Trowel a patch kit onto the surface.

Clean Any Large Gunk First and Use an Oil Cleaning Solution

The goal in cleaning and preparing the surface is to create a good contact surface for the patching material. Use a putty knife and de-greaser to get rid of any surface gunk first. After this step, you will be scuffing the surface with an abrasive material. So in this step, you will remove any floor crud that would not easily come up by scrubbing it with a wire brush. It’s OK to leave an embedded oil stain. You can de-grease that if it’s still there after hitting the area with abrasives.

Hit the Area with Abrasives to Remove Surface Concrete

Next, use an abrasive tool to rough up the surface area that you’re going to patch. Remove surface concrete to expose any weak spots and to create a fresh surface to patch. The tool depends on the low spot situation. You can use a wire brush, a grinder and a buffer, or a concrete etching acid substitute product.

Wire Brush Pros and Cons

Wire brushes work well on very small area. “Small” is whatever size you’re comfortable attacking with your own elbow grease. The wire brush abrasion method is cheap, requires only a simple brush, and gets the job done. Attacking larger low spot jobs with a wire brush will be time-consuming.

When to Use a Grinder and Buffer

DeWalt angle grinder and caseThe grinder-buffer concrete resurfacing method automates the re-surfacing step. You’ll need an angle grinder with a diamond turbo cup wheel, a garage vacuum cleaner, and an optional vacuum shroud. So it’s a lot of equipment.

An angle grinder is a hand-held power tool that spins a disc that literally grinds away and removes the surface concrete. If you’ve never used an angle grinder before, this might not be the step for you. Angle grinders throw sparks and debris. You absolutely MUST wear goggles and gloves. And you’ll definitely want a vacuum shroud to send the debris directly to the vacuum cleaner. Also be aware that if you have little experience with grinders, it is not as easy as it seems. You can easily gouge your floor if you hold the grinder unevenly.

Home Depot floor maintainerFor very large areas consider using a floor buffer (also called a “floor maintainer“). A floor buffer is a long-handle powered floor polisher. Think of your high school janitor pushing a machine to clean the floors. That’s a floor buffer. It’s also called a “floor maintainer.” Floor buffers aren’t standard household equipment. Rather than buy one, you would most likely rent a floor buffer from your hardware store.

You will need to rent a Diamabrush concrete prep tool that attaches to the bottom of the buffer or a Diamabrush removal tool that is designed to scrape up adhesives and other gunk that is stuck to the floor. If there are areas near the wall or near corners you will also need the grinder.

Use the grinder and buffer when you know you will be comfortable with these tools. They’re not necessarily easy to use. But they do clean out the area by grinding away at the surface. They will help remove a messy oil stain that might prevent the patch from sticking. They also automate the process should you need to repair multiple low spots.

Grinding is more expensive than etching as it requires more tools. But it’s faster and potentially more accurate, so long as you maintain a good hold on your tools. Grinding produces a lot of debris, even when using a vacuum shroud. Wear safety goggles and gloves.

Rust-Oleum Clean & Etch for ConcreteWhen to Use a Concrete Etcher (Acid Substitute)

Etching the garage floor surface requires much less equipment than the grinder-buffer method. People used to use muriatic acid to resurface low spots. Muriatic acid is dangerous to touch and breathe. The marketplace responded with non-toxic substitutes to etch your garage floor. Don’t use acid if you can help it. The substitutes work really well without the associated burning and toxic dangers.

While etching with muriatic acid won’t remove oil, acid-substitute etchers just might work here. If the area is still oily after removing surface gunk, treat the concrete with an oil de-greaser before etching the surface. Learn more about Oil Eater below.

Etching is an alternative to grinding and buffing when you have a large low spot area to fix. Etching is cheaper than grinding and buffing, but a little bit more expensive than using a wire brush. Etching does not require lots of tools, and does not shoot sparks and concrete debris.

You do need to babysit the etching process in order to remove the etcher before it removes too much concrete. Also never let the etching solution dry on your floor. Rinse three times after the etcher has done its work.

Garage Low Spot Fix Step 1: Cleaning and Preparation

Clean the garage floor. You want to make sure the concrete surface is free of sealers, oils and liquids. Anything that will prevent bonding must be removed. The major prep will be done with either grinding or etching. But before you can do that, you need to remove oil and any other greasy material from the floor. Use warm and soapy water or Tide detergent to clean the floor. If oil still remains, use an oil remover.

Tide detergentRumor has it NASCAR uses Tide laundry powder to clean oil stains. I doubt their stains have been around as long as yours have. If you want to give it a try, go for it. You can always do laundry with the rest.

You will need: a bucket; Tide laundry powder; water (warmer is better); a stiff bristle brush; and a hose.

Mix up a strong batch of powdered Tide. Pour the mixture onto the stain. Then add more dry Tide powder. Brush the mixture and powder into the stain. Let it soak for 15 minutes. Don’t let it dry. If it starts to dry before 15 minutes, add more solution to the floor. Then hose off the soapy water.

Perhaps an easier option is to mop the area with a warm solution of dish soap water. You will need: a bucket; dish soap; warm water; a stiff bristle brush; and a hose. Brush the soap solution into the garage floor and rinse with a hose.

Oil Eater de-greaserIf you still have an oil stain in the low spot area, treat it with Oil Eater. Oil Eater is a nonabrasive, nontoxic, non-flammable, non-corrosive, non-acid, non-petroleum-based product. You can use it on your garage floor, but you can also use it on clothes, engine parts, carpet and upholstery. Oil Eater is so safe that it’s legal to use it on commercial food surfaces. Oil Eater has a great reputation for quickly removing oil stains. Check it out on Amazon: Oil Eater.

Garage Low Spot Fix Step 2 Option A: Grind and Buff

Do Step 2A (Grind and Buff) or Step 2B (Etching), not both.

WARNING: Be extremely careful with power tools. I know a handyman who lost his grip working on a stubborn screw. He broke his wrist. Over a screw. Don’t take safety lightly.

After you remove excess oil and dirt from the floor, grind and buff the low spot. This will remove excess oil, and prepare the surface to accept patching material.

Grinding is a lot of work. If you’ve never used an angle grinder before, consider using an acid-substitute etcher instead. This tool throws a lot of sparks and debris. It’s messier than etching, and requires a lot more equipment. Also it is easy to damage your floor by applying too much pressure or tilting the tool even slightly. Don’t underestimate the torque a grinder can produce. Holding it steady can be tiring.

You will need the following tools, which Home Depot rents out:

  • a 4.5 in. angle grinder (to get into tight spots and corners)
  • a 4.5 in. crack chasing angle grinder disc, also called a “diamond grinding cup wheel.” If you already have a 4 in. angle grinder, this is a good 4 in. crack chasing disc.
  • a 5 in. universal dust shroud (captures angle grinder output)
  • a floor buffer (also called a “floor maintainer” (to prepare the rest of the low spot)
  • if the floor is covered in epoxy or adhesive, a floor buffer Diamabrush concrete prep tool (video) and water for its operation
  • if the floor is not covered in epoxy or adhesive, a floor buffer Diamabrush concrete removal tool (video)
  • a garage vacuum cleaner
  • eye protection
  • face mask

Attach the vacuum shroud to the angle grinder and the garage vacuum cleaner. Wearing eye protection, gloves and a face mask, use the angle grinder to remove the top layer of the low spot. The angle grinder is especially useful near corners and edges. The vacuum shroud is essential; otherwise the concrete will fly all over the garage. It is cement. It will harden and leave a big mess.

Next, use the floor buffer to remove the rest of the low spot surface. Use the Diamabrush concrete prep tool. If you have adhesives or old epoxy stuck to your floor use Diamabrush concrete removal tool. This removal tool is just for removing adhesives or old epoxy. It’s not for cleaning oil stains.

We’ve covered only the basics here. Using a grinder and buffer to prep the low spot includes a lot of steps. There’s a great article about it on All Garage Floors that you should read if you’re going down this path.

Garage Low Spot Fix Step 2 Option B: Etch

Do Step 2A (Grind and Buff) or Step 2B (Etching), not both.

After reading about the messy work that goes into grinding, I bet you are hoping etching is easier. It is. Somewhat. The process requires fewer tools and less labor, but it can be messy and toxic. But if you use a garage floor acid-substitute etcher, it’s pretty easy compared to grinding and buffing.

Etching requires a stiff bristle broom, squeegee or mop to apply; water to mix and to rinse; and a whole lot of scrubbing. You might also like to use a large spray applicator for larger areas.

So long as your garage floor doesn’t have epoxy or stubborn substances on it, you can clean it with an etching cleaner.

Best concrete etcherOne of the best garage floor preparation products is Rust-Oleum Clean & Etch. It’s a huge advancement over traditional acid etching products. It’s non-toxic, doesn’t put off fumes, and is not dangerous.

Yes, you could use muriatic acid to prep the floor, but Clean & Etch is safe for you, your pets and your plants. It also gets rid of oil stains. (Check it out on Amazon.)

To clean the area to be patched, you will mix the product with a gallon of water inside a plastic watering can. You will scrub the solution into the floor until it stops fizzling. You will rinse it off a total of three times before you’re done.

You have to have a way to get rid of the water. One of the reasons I like Clean & Etch is because it’s safe to rinse the cleanser out of the garage. When rinsing an acid wash, you risk sending acid onto your lawn and plants.

Acid Product Alert! Don’t Use Acid. Use This Instead.

WARNING: If you choose to etch the area, use an acid substitute. Acid is dangerous to humans, pets and properties. Many products and tools for resurfacing and preparing are designed with professionals in mind. I have tried to find the most user friendly solutions to fix your concrete low spots. Be careful and consider getting a professional. Don’t use muriatic acid. The substitutes work well without the dangers.

If you’re etching with acid, it can produce dangerous fumes. A big hint that the product contains acid is if they tell you not to use metal. So if you do use acid, do NOT let it sit, do NOT breathe the fumes, and do NOT let it touch anyone’s skin. Also, never let acid touch metal.

WARNING: Do not just pour acid or the acid substitute on the garage floor. The acid will leave you with a nice hole to fix. Use a sprayer or a watering can instead. You want to apply an even coating not a puddle. Do not apply chemicals or acids in an enclosed space.

Low Spot Repair Step 3: Resurface the Low Spot

Before you begin, think about your path. Leave yourself an exit! You don’t want to cement yourself into a corner.

To resurface the low spot, you will need the Patch & Repair Kit; cardboard for mixing; a trowel for mixing and applying; rubbing alcohol to clean the trowel.

You cannot fix a concrete low spot with more concrete. It won’t stick. You must get a concrete resurfacing product. I like the Rust-Oleum Concrete Patch & Repair Kit. It’s very strong and will fill a spot up to 1/2 in. deep. The Patch & Repair Kit will dry to a medium gray color. After 8 hours, you can coat it with a sealer or paint it. Check out the Patch & Repair Kit here.

The Patch & Repair Kit is a two-part epoxy. You will need a throw-away surface to mix it on, such as a piece of wood or cardboard. You’ll also need water, two pails, and a grading rake or squeegee. Get a helper if you can, so one person can mix batches while the other applies them.

Follow the package instructions as there are different mixing ratios. Make a denser mixture for deeper areas. After the deep areas are filled, mix a more liquid batch. Pour it onto the area, then rake or squeegee it evenly across the floor.


Best Tools for Fixing Garage Floor Low Spots

.

More Garage Floors

.

More Garage Maintenance

.