Tech Tip: How to Maximize Spray Foam Adhesion to Any Substrate

Proper preparation and technique are what make spray foam bond firmly to wood, concrete, metal and other surfaces. The rules below follow the processing requirements printed on every ThermoSeal technical data sheet, which also govern warranty coverage, so treat them as the standard rather than a suggestion.

The Short Version

• Clean, dry, and roughened if necessary. If a surface couldn't be painted as it is, it isn't ready for foam either.
• Substrate at least 10°F above the dew point. This is the surface temperature, not the air temperature.
• Moisture content: wood below 18%, concrete below 10%.
• Inside the product's temperature limits for both the substrate and the air.
• When in doubt, spray a test patch before you foam the whole area.

Surface Preparation by Substrate

Always start with a substrate that is clean, dry and free of contaminants. Remove dust, dirt, grease, oil and loose debris from the area to be foamed. Any damp surface causes the A side (isocyanate) to react with the moisture instead of bonding to the substrate, which produces poor cell structure and weak adhesion.

Wood

Studs and sheathing must be dry and free of sawdust. Check with a moisture meter and do not spray wood above 18% moisture content; if it reads higher, let it dry or run dehumidifiers first. Wipe away mill oils, and if the wood is very smooth or has flaking paint, scuff sand it and remove the loose paint so the foam bonds to solid wood rather than to a weak layer.

Concrete and Masonry

Concrete must be fully cured and below 10% moisture content. Brush, blow or vacuum away dust and laitance (the loose powder on the surface). Clean form-release oil or grease with a suitable degreaser or light abrasive blasting, because oil repels foam. On very porous or chalky masonry, a primer will lock down loose particles.

Metal

Remove rust, scale, oil and condensation. Wire-brush or sand off rust, and wipe away the oily film often found on new steel and galvanized metal with a solvent cleaner. Smooth factory finishes on aluminum, stainless and galvanized steel reduce adhesion, so lightly abrade them or use an etching primer.

Watch for condensation: metal that is cooler than the air attracts moisture. And watch the opposite extreme too. Metal in direct sun can exceed 150°F, which makes foam react too fast. Spray metal buildings early in the day when the substrate is coolest.

Painted and Other Surfaces

Foam only sticks as well as whatever it is sprayed on. Scrape off peeling or chalking paint, or prime over it. For unusual substrates, spray a test patch and check with ThermoSeal technical support before committing to the whole area.

Primers and Surface Treatments

The right primer can greatly improve adhesion on difficult substrates and seal surfaces against moisture.

• Steel and iron: an anti-corrosion primer both inhibits rust and gives the foam a surface to grip.
• Galvanized steel, aluminum and copper: an etching primer designed to bite into non-ferrous metal.
• Concrete, brick and stone: an acrylic primer locks down dust and helps block residual moisture. A dark-tinted primer also absorbs sunlight and warms a cold substrate.
• Wood: clean, dry wood usually needs no primer. Old or weathered wood can benefit from a coat of acrylic primer.

Always let a primer dry fully according to its manufacturer's directions before spraying foam over it.

Temperature, Dew Point and Moisture

Every ThermoSeal technical data sheet requires the substrate to be at least 10°F above the dew point. Measure the surface itself with an infrared thermometer, and compare it to the dew point for current conditions. If the gap is less than 10°F, moisture can condense on the surface while you spray, even when nothing looks wet.

Each product also has minimum substrate and ambient temperatures:

If the area is too cold, warm the space with an indirect-fired heater before spraying and shut it down while you spray. Direct-fired heaters add water vapor to the air and are an ignition source. Never use an open flame to warm a substrate.

Humid attics and crawlspaces benefit from fans or dehumidifiers before spraying. Outdoors, use wind screens where needed, and check nearby surfaces for overspray dust before you foam them.

Common Mistakes to Avoid

• Spraying wet or frosty surfaces. Especially common on early-morning and cold-weather jobs. Verify the surface is dry, every time.
• Leaving dust, dirt or oil behind. Foam sticks to the dust, not to the wall. Vacuum cavities, wipe dusty wood and degrease oily metal.
• Skipping primer on difficult substrates. Shiny metal and previously painted surfaces often need scuffing or priming.
• Spraying outside the temperature limits. Foam applied too cold or too hot may not cure correctly and can shrink or pull away.
• Off-ratio foam mistaken for an adhesion problem. Foam that is friable or curls away as it sets usually points to the equipment, not the surface. See our guide to troubleshooting spray foam pressures.
• Poor timing between closed cell lifts. Spraying the next lift before the previous pass has cooled stacks heat and causes delamination. Waiting too long lets dust or an aged skin form. Follow the lift and recoat guidance on the product data sheet.

Fixing Adhesion Problems in the Field

Foam peels off right away

Stop spraying and find the cause before continuing. The usual culprit is moisture or contamination. Dry the area thoroughly, scrape off the unbonded foam, re-clean if needed, and consider priming before you resume.

Foam won't stick to metal

Remove the loose foam, then grind or sand the surface to create a profile and apply a suitable metal primer. Make sure the metal is dry and above the dew point, then spray a small test area to confirm the bond before continuing.

Layers separating from each other

Cut out or sand off the loose layer, remove any glossy skin from the layer beneath, and re-spray within the product's recoat guidance.

Foam shrinks or cracks after it looked fine

This usually points to a ratio or temperature problem rather than the substrate. Check proportioner settings, chemical temperatures and pressures, then spray a test slab to confirm good foam. Remove the defective foam and re-apply; spraying over bad foam will not fix it.

The Bottom Line

Thorough surface prep, primers where needed, and staying inside the dew point and temperature limits will give you foam that stays put and an air seal that lasts. When a problem does show up, work through the causes above in order: moisture first, then contamination, then temperature, then equipment.

Frequently Asked Questions

Will spray foam stick to metal?

Yes, when the metal is clean, dry, free of rust and mill oil, and at least 10°F above the dew point. Smooth metals such as galvanized steel, aluminum and stainless need to be lightly abraded or coated with an etching primer first so the foam has something to grip.

Will spray foam stick to concrete?

Yes. The concrete must be fully cured, below 10% moisture content, and free of dust, laitance and form-release oil. New concrete can hold moisture for weeks after it is poured, so verify it with a moisture meter rather than by touch.

Will spray foam stick to plastic?

It depends on the plastic. Foam does not bond reliably to polyethylene, which is why poly sheeting is used for masking. For PVC and other plastics, abrade the surface and spray a small test patch before committing, or call ThermoSeal technical support.

What does spray foam not stick to?

Anything wet, frosted or condensing; surfaces with oil, grease, dust or release agents; loose or flaking paint; and low-friction plastics such as polyethylene sheeting. Foam also bonds poorly to its own surface once that surface has aged, collected dust or sat beyond the recoat window.