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How to Sand Thinset Mortar- 3 Best Practices

Can you sand thinset mortar?

Yes, you can.

Your tiling project may require sanding thinset for a smooth finish.

Thinset mortar contains cement that dries very quickly. Dried thinset is tough stuff. So, sanding the dried thinset takes some work and effort, but you can do it.

Wondering how to smooth out dried mortar?

In this post, I’ll explain how to sand thinset.

Without further ado, let’s dive in.

Here I’ll discuss three best ways to sand thinset.

how to sand thinset
How to Sand Thinset

Safety Measures During Sanding Thinset

  1. No matter which method you follow, make sure that you wear a dust mask or respirator ( N95 respirator is recommended). Dust contains silica and prolonged exposure to dust may cause lung disease. A dust mask will prevent the dust particles to reach your lungs.
  2. Wear protective goggles to ensure that no flecks will get in your eyes. If any flecks get into your eyes, flush immediately and repeatedly with water. Don’t forget to consult a physician.
  3. Freshly mixed mortar, grout, concrete, or cement may cause skin injury. Try to avoid contact with the skin. Wash exposed skin immediately with clean water.
  4. Vacuum or wipe away the dust every few minutes of sanding. Thoroughly vacuum or wipe the area when finished.

Method 1: Grit Sandpaper and Wood Block Method

If you need to sanding thinset of a small area, you can use grit sandpaper to remove it. Sandpaper works well for smoothing. For this, you’ll need sandpaper anywhere around 24 to 80 grit.


Take a wooden block and wrap the coarse sandpaper around it. This will help you protect your hand from scrapes and apply more pressure when sanding.
First, sand up and down. Then sand side to side.

Method 2: Black Block or Rub Brick/ Carborundum Rubstone Sanding Method

Black block or rub brick/ stone is one type of shaped chunk of lava rock. In general, it comes with a handle on it. You can find it at most hardware stores. This stuff is pretty amazing for sanding thinset.

rub brick

First, take a stiff 3-inch putty knife and make a light pass to remove any loose thinset. If you find any large bumps or high points of the excess thinset, instead of using coarse paper follow the rub brick or black block method. Carefully, grind the thinset down.

3 inch stiff putty knife

Sand up and down and then side to side. You have to apply force while sanding but this method is controllable. Conveniently, it doesn’t raise a lot of dust.

Method 3: Power Grinder Sanding Method

A handheld power grinder is the best option for sanding thinset mortar of a large area.

best angle grinder

First, read the manufacturer’s instructions for your power grinder.

Start sanding with 24 to 80 grit sandpaper.

Pass the grinder back and forth over the thinset until it is perfectly level and smooth.
Vacuum up or wipe away the dust before installing the tiles.

If you plan to paint over the thinset, sand the area again with coarse-grit sandpaper of 80 to 100 grit. When finished, clean the area thoroughly to remove all dust so that you get a smooth paint finish.

This is all about sanding thinset mortar.

Which is the best option to sand old thinset for preparing it for new tiles?

The best way is to grind it down. You can use a floor grinder to get the job done. Pass the grinder back and forth over the old thinset mortar. Repeat the process until you get a smooth and level surface. Once done, you can directly tile on top of it.

How long for thinset to dry before sanding?

All of it depends on the humidity and ambient temperatures. However, the process may take 12 to 24 hours for a minimum cure.

Can you paint thinset?

YES- you can paint over thinset mortar.