Concrete

Grinding Off Thinset, Mastic and Glue Residue

Getting thinset, mastic and glue residue off concrete: testing first, choosing between PCD, metal bonds and floor-machine attachments, and handling the dust.

Updated · 2 min read

Leftover thinset and adhesive have to come off before most new floors go down. The tool depends on what's there: hard thinset grinds, gummy adhesive clogs diamonds, and black mastic has to be tested before anyone grinds it.

Test first, then control silica

Old black mastic and cutback can contain asbestos. Grinding asbestos-containing floor material makes it regulated under EPA NESHAP[2] and is prohibited by OSHA outside specific controls.[1] See Cutback adhesive and asbestos ID. Once cleared, grinding still releases silica: follow OSHA's silica controls.[3]

Match the tool to the residue

Find the residue, then the tool.

Residue First choice Then
Thinset ridges Scrape the ridges, then 16 to 30 grit metal bond on a grinder[4] Finer grit to the required profile
Thick or gummy adhesive, epoxy PCD tooling[4] Metal bond to clean up
Thin, well-bonded adhesive film Mastic-removal attachment on a 17" floor machine, or 16 to 30 grit metal bond Vacuum and check
Edges and corners 7" hand grinder with a shroud and vacuum

Step by step

Once the lab clears the adhesive, work from the most aggressive tool down.

  1. Get the lab report for the adhesive. Don't start until it's clear.
  2. Scrape off everything that will come off with a blade. It's faster and cheaper than grinding. See Scraper blade selection.
  3. Set up dust collection sized to the machine before the first pass. See Dust extraction.
  4. Make the first pass with the most aggressive tool the residue needs.
  5. Check what the new floor requires. Many adhesive and patch makers want residue removed to clean, sound concrete; some allow a thin, well-bonded film. Stop when you meet it.
  6. Grind edges with a shrouded hand grinder.
  7. HEPA-vacuum, check profile (ICRI CSP) and flatness.

Common mistakes

Most wasted time and tooling on this job comes from skipping steps or using the wrong tool.

  • Grinding before scraping. You'll burn through tooling removing material a blade would have taken.
  • Metal bonds on gummy adhesive. The segments load up and polish. Use PCD.
  • Running the hand grinder without a shroud. Indoors, Table 1 requires a shroud and 25 cfm per inch of wheel diameter.[3]
  • Over-grinding. You only need the profile the new product calls for.

Tools we carry

These are the tools we carry for residue removal.

Regulations and references

  1. OSHA 1926.1101, Asbestos (construction)
  2. EPA Asbestos NESHAP, 40 CFR 61 Subpart M
  3. OSHA 1926.1153, Silica, including Table 1
  4. National Flooring Equipment: Grinder tooling guide (bond, grit and PCD)

Linked PDF documents are archived copies saved by PreFloor Tools so they stay available if the publisher moves or removes them. Check the publisher's website for the latest revision.

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