Sanding vs. Grinding: Preparing Previously Painted Concrete

When you are faced with a floor that is already painted, you have a critical decision to make. Do you rough it up and paint over it? Or do you strip it back to the bare concrete and start fresh?

It is the difference between a "Refresh" (Sanding) and a "Reset" (Grinding).

Make the wrong choice, and you will waste hundreds of pounds. If you grind a floor that only needed sanding, you’ve wasted time and money. But if you sand a floor that needed grinding, your new Industrial Floor Coating System will peel off in sheets within months.

Here is the professional guide to preparing a previously painted floor, and how to decide which method you need.

The Cross-Hatch Test

The first step in the process is testing the floor. The goal here is to determine what level of preparation is needed to get the floor ready for a fresh coat of paint.

The best way to figure this out is a Cross-Hatch Adhesion Test. This test confirms how well the previous coat of paint is stuck down to the concrete. If the current coat of paint is barely hanging on when you paint over it, youre building on top of a weak foundation. This will lead to a much shorter lifespan for the new coat.

To perform the Cross-Hatch Adhesion Test:

1. Cut: Take a sharp craft knife and cut a hashtag (#) pattern into the old paint. Cut deep enough to hit the concrete.

2. Tape: Stick a piece of strong duct tape over the cuts. Rub it down hard.

3. Rip: Pull the tape off aggressively.

The Results:

• If paint comes off on the tape: The old coating is failing. You cannot paint over it. You must grind it back.

• If the tape is clean: The old paint is bonded solid. You are safe to recoat. All you need to do is lightly sand the previous coat, this provides a good surface for the new paint to bond to.

Option 1: Sanding (The "Refresh")

Goal: To create a "Mechanical Key" without removing the coating.

If your old floor is sound (it passed the test) and you are simply changing the colour or refreshing a tired surface, you do not need to remove the paint. However, you cannot paint over glossy surfaces. You need to "de-gloss" them.

• The Tool: An industrial floor buffer or orbital sander with 80–120 grit abrasive pads.

• The Process: Scuff the entire floor until it looks dull, white, and chalky. You aren't trying to see concrete; you are just trying to kill the shine.

• The Primer Rule: If you are sanding and recoating, you generally do not need a primer. You can apply your floor paint directly to the sanded surface.

Option 2: Grinding (The "Reset")

Goal: To obliterate the old coating and expose bare concrete.

You must choose this option if:

1. The old paint is flaking: Painting over flaking paint is like building a house on a swamp. The new paint will just lift the old paint faster.

2. Incompatibility: You want to apply a heavy-duty Epoxy over a cheap, weak single-pack acrylic. (The epoxy solvents will eat the acrylic).

3. Unknown History: You have no idea what is on the floor and don't want to risk a reaction.

• The Tool: A walk-behind concrete grinder with aggressive "PCD" (Polycrystalline Diamond) shoes or coarse diamond segments.

• The Process: The grinder shreds the old coating and creates a fresh surface profile on the concrete underneath.

• The Primer Rule: Because you have exposed bare concrete, you must treat this as a new floor. You must vacuum thoroughly and apply a primer before you apply and paint.

The "Patchy" Trap

What if 80% of the floor is sound, but 20% is flaking?

Do not try to "spot grind" just the bad bits and sand the rest. This creates "ridges" in the floor that will show through your new paint. If significant areas are failing, it is safer to bite the bullet and grind the whole floor. It ensures that your new floor paint has a uniform surface to bond to.

Conclusion

If the previous coat is loose, your new coat is loose. Don't be lazy with the prep. Do the tape test today. If that tape comes up clean, grab a sander. If it comes up dirty, book the grinder.