Moisture testing and prep before coating a garage floor

Most coating failures on garage floors start under the coating, not in it. Peeling, bubbles and patches that lift after the first winter usually trace back to moisture, contamination or poor surface profile. A consistent prep routine is the cheapest insurance an epoxy crew can buy.

This is general guidance. Always follow the data sheets and installation instructions for the products you use, since limits and methods vary by manufacturer.

Why moisture matters

Concrete is porous. Moisture can move up through a slab from the ground below, especially on slabs without a vapour barrier. When a non breathable coating covers the surface, that moisture can build pressure under it and cause blisters or loss of adhesion. Garages in Ontario also see melting snow, road salt and big temperature swings, which add stress to any coating.

Quick screening: the plastic sheet test

Tape a square of clear plastic sheet, roughly 18 by 18 inches, tightly to the floor on all four sides and leave it for at least 16 hours. If you see condensation under the plastic or the concrete darkens, there is a moisture concern worth testing further. This method is described in ASTM D4263. It is qualitative, so treat it as a warning sign, not a measurement.

Measuring moisture

Two common quantitative methods are used on slabs:

• Calcium chloride test (ASTM F1869): measures the moisture vapour emission rate from the surface over a set period, reported in pounds per 1,000 square feet per 24 hours.
• In situ relative humidity probes (ASTM F2170): probes are placed in drilled holes at a set depth to measure relative humidity inside the slab.

Compare the results to the limits on the primer or coating data sheet. If the readings are above the limits, a moisture mitigating primer or vapour barrier system designed for that purpose may be needed. Price that as a separate line item and explain it to the homeowner before the job, not after.

New concrete needs time

Fresh concrete releases a lot of moisture as it cures. A common rule of thumb is to wait at least 28 days before coating, but the real answer depends on the mix, the weather and the coating system. Test rather than assume.

Check for old sealers and contamination

Sprinkle water on several spots. If it beads up or sits on the surface instead of darkening the concrete, there may be a sealer, paint or contamination that will block adhesion. Oil and other stains also need attention. Degrease, then grind or remove whatever is on the surface until clean concrete is exposed.

Grind to the right profile

Most garage floor systems call for mechanical preparation with a diamond grinder rather than acid etching. The goal is a clean, open surface with the concrete surface profile (CSP) that the coating manufacturer specifies, often in the CSP 2 to 3 range on the ICRI scale for thin film systems. Pay attention to the edges and corners with an edge grinder, and vacuum thoroughly with a HEPA equipped vacuum when grinding is done.

Repair cracks, spalls and joints

• Open up and fill cracks with a product suited to the coating system
• Patch pitting and spalled areas, which are common near the garage door where salt collects
• Decide with the homeowner how control joints will be treated and note it on the quote
• Explain that moving or structural cracks may telegraph through any coating over time

Watch temperature and dew point

Coatings cure slowly in the cold and can fail if moisture condenses on the slab. A widely used guideline is to keep the slab temperature at least 3 degrees Celsius (5 degrees Fahrenheit) above the dew point during application and cure, and within the temperature range on the data sheet. Unheated Ontario garages in late fall and winter can drop out of that range quickly. Use a surface thermometer and a hygrometer, and reschedule rather than gamble.

Document what you found

Take photos of the floor before prep, note test results and any repairs, and keep them with the job. If an issue ever comes up, you will have a record of the conditions and what you did about them. It also helps you price similar floors more accurately next time.

Build prep into your quotes

Prep is where quotes get underpriced. List moisture mitigation, coating removal and repairs as clear line items so the homeowner understands why they matter. Poxio, an app from 22G Media with a free forever plan, lets you set those extras once and add them to any quote in a tap, with job photos saved alongside.