Preventing Blisters in Sealants

Blisters in sealants aren’t uncommon, but they can be avoided and are fairly simple to fix. Following is a brief discussion of their causes and solutions.

Causes of Blisters

The basic cause of any blister is a build-up of gas between the substrate — a log, wood siding, concrete slab, etc. — or backing surface and the sealant. It occurs during the early curing stages when the sealant is soft but has formed a skin on the surface. Heat, usually in the form of direct sunlight, causes the gas to expand. As the gas expands, it pushes the sealant out, creating a blister. This gas can come from water, wood sap, or even the backing material.

What Circumstances Create the Gas?

Scenario #1 – Trapped Water Vapor:

As water-based sealants cure, they give off water vapor. Any physical condition that traps the resulting water vapor could cause a blister, especially if the substrate and the sealant are heated by direct sunlight.

Scenario #2 – Dead-End Crack:

Dead-end cracks in the substrate where moisture vapor accumulates can expand when heated.

Scenario #3 – Pitch Pockets:

Logs in particular often have what is termed «pitch pockets.» These are areas of concentrated pitch, or sap, sometimes containing several gallons! (Pitch pockets of 50 or more gallons are known!)

This sap (or its vapor) will follow cracks in logs, coming out of the log — almost anywhere. Sap has a volatile component and, when heated, forms a gas which causes blisters.

Preventing Blisters

The basic cause of any blister is a build-up of gas between the substrate — a log, wood siding, concrete slab, etc. — or backing surface and the sealant. It occurs during the early curing stages when the sealant is soft but has formed a skin on the surface. Heat, usually in the form of direct sunlight, causes the gas to expand. As the gas expands, it pushes the sealant out, creating a blister. This gas can come from water, wood sap, or even the backing material.

Control the Heat:

1. Always apply sealants when it’s shady and cool. Avoid application to surface temps that are over 90°F whenever possible.
2. When necessary (70%+ humidity, air temps above 85°F), protect surfaces from direct sunlight for a few days by covering with a light-colored tarp, cloth, or plastic. Leave plenty of room for ventilation.
3. Prime or stain the substrate first. This can help to close some of the dead-end cracks first.
4. Use blunt tools to install closed cell backing materials, being careful to avoid punctures.
5. When applying sealants over flat insulation (foam, bead board or foil-faced foam board), surface cracks, holes or dents should be covered with clear packing tape.

Control the Backing Material:

1. Use blunt tools to install closed cell backing materials, being careful to avoid punctures.
2. When applying sealants over flat insulation (foam, bead board or foil-faced foam board), surface cracks, holes or dents should be covered with clear packing tape.

Fixing Blisters

When Fresh:

1. Puncture the surface skin to release the gasses.
2. Push the sealant down and back into place.
3. 2 to 3 days later, apply a skim coat over the blistered area and smooth out to blend it in.

WHEN CURED / DRIED ALL THE WAY THROUGH:

1. Cut out the dry blister with a razor blade.
2. Apply more sealant.
3. If it shrinks a bit and isn’t quite flush with the rest of the bead, apply another skim coat 2-3 days later to ensure the whole bead matches.