You should avoid sealing driveway cracks when the concrete is hot from sunlight because heat causes sealant bubbling and poor adhesion.

When concrete is hot, volatile components in the sealant expand rapidly, creating bubbles and pinholes that prevent a continuous bond to the substrate. Measure surface temperature with an infrared gun held perpendicular about six inches away and average readings across the area to avoid misleading hotspots. Aim to apply sealant only when concrete surface temperature is between forty degrees Fahrenheit and one hundred ten degrees Fahrenheit; if the surrounding air is cooler but the slab is sun-warmed, wait. Work in shaded or cloudy conditions whenever possible, or schedule work for early morning or late afternoon when the slab has cooled; this reduces evaporation and improves flow into cracks. Clean and dry the cracks before application, removing loose material with a stiff brush and using a blower or compressed air to remove dust, because contaminants combined with heat worsen adhesion failures. When tooling, press the sealant into the crack and smooth it to eliminate trapped air and to ensure the material contacts vertical faces inside the joint. Allow full cure times specified by the manufacturer while keeping traffic off the area and protecting it from sudden temperature swings that can cause stress in the new sealant.

The three most common problems are bubbling during cure, poor adhesion after a few weeks, and premature cracking of the sealant. Bubbling is usually from applying on a hot surface, so cool the slab by working in shade or waiting until evening and verify with an infrared gun. Poor adhesion is often caused by surface contamination or moisture, so sweep, brush, and blow out cracks and allow any dampness to evaporate before sealing. Premature cracking comes from using the wrong product for the expected joint movement or from applying when the material cannot properly flow, so choose a flexible polymer sealant rated for pavement joints and follow manufacturer temperature and tooling guidance.

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