The right primer for self levelling compound is decided by the substrate, not by the brand on the tin. Porous concrete usually takes an acrylic primer, often in two coats, because the first coat soaks in and the second builds the film the leveller bonds to. Smooth, power floated concrete, calcium sulphate screed and timber need a primer the product data sheet names for that surface, and an epoxy primer where moisture or a weak surface is the problem. A primer is not a damp proof membrane: if the slab is damp, test it and deal with the moisture first, because no primer coat will hold back rising damp on its own.
Introduction to Self Leveling Compounds
Self leveling compound, also known as self leveling underlayment or self leveling concrete, is a flowable material used to create a smooth, flat surface prior to installing floor coverings like tile, vinyl, or wood. As the name suggests, self leveling compounds level themselves out to create an even surface without the need for tools or manual labor to spread and smooth.
Benefits of Using a Primer
While self leveling compounds provide a convenient way to create a flat, smooth subfloor, using a primer underneath provides additional benefits. Some key reasons to use a primer include:
Promotes Adhesion
Primers enhance the adhesive bond between the self leveling underlayment and the substrate underneath. This prevents shifting, crumbling edges, cracks, and other bond related issues.
Reduces Moisture Issues
Primers create a protective barrier to reduce moisture transfer between the substrate and self leveling mix. This prevents moisture issues like bumps, craters, and de-bonding.
Standardizes Absorption
Variations in substrate porosity can lead to an uneven cure and moisture pockets. Primers help standardize absorption rates for consistent drying.
Choosing the Best Primer
With an understanding of why priming is important prior to pouring self leveling underlayments, now let’s discuss how to choose the right primer:
Type of Substrate
Primers must be compatible with the substrate they are applied to. Most self leveling floor primers are formulated for concrete floors, but specialty primers exist for wood, OSB, and metal substrates.
Resin Type
Common resin types include acrylic, epoxy, and urethane. Acrylics provide basic sealing and adhesion, epoxies offer superior bonding and moisture protection, while urethanes provide the best durability and chemical resistance.
Key Qualities in a Self Leveling Floor Primer
| Feature | What to Look For | Benefits |
|---|---|---|
| Substrate Compatibility | Compatible with concrete, wood, OSB, metal, etc. | Ensures proper adhesion to the substrate beneath self leveler |
| Resin Type | Acrylic, epoxy, urethane or other resin | Provides moisture protection, penetration, and adhesion qualities |
| Solids Content | Higher % solids content | Builds a thicker film layer with better sealing |
| Viscosity | Thin primers offer better penetration | Deep penetration fills pores on porous substrates |
| VOCs | Low VOC primers are safer | Important for indoor air quality, especially commercial sites |
| Application Method | Roller, brush, or spray apply | Matches preferred application method |
| Drying Time | Quick drying primers speed projects | Allows self leveling to be poured shortly after priming |
| Second Coat Capable | Single coat or re-coatable | Determines if a second coat should be applied to very porous concrete |
Top Self Leveling Primer Reviews
Based on the criteria above, here are reviews of 5 top rated primers for use beneath self leveling underlayments:
Mapei Planicrete UA
Best moisture barrier protection
Mapei Planicrete UA is a 2 component, high solids epoxy primer for porous and non-porous substrates. It creates a thick film to prevent moisture issues beneath self leveling underlayments.
Key Features:
- Excellent adhesion and porosity control
- Reduces pH and moisture transfer
- Can be applied to 7-day old concrete
Pros
- Prevents virtually all moisture-related issues
- Compatible with nearly all substrates
Cons
- Very expensive
- Strong odor during application
KILZ Adhesion High-Bonding Interior/Exterior Primer
Best budget option
KILZ Adhesion is a fast-drying, water acrylic-latex primer and sealer. It improves adhesion on multiple surfaces at a very affordable price point.
Key Features:
- Bonds to glossy surfaces without scuff sanding
- Low odor and VOC content
- High build film resists suction
Pros
- Extremely economical option
- Low odor for interior use
Cons
- Lesser moisture protection
- Not for exterior porous substrates
Euclid Chemical Super Flo-Coat
Best high flow primer
Super Flo-Coat is a thin epoxy acrylic primer with exceptional penetration and sealing properties. It has very low viscosity to penetrate deep into porous concrete.
Key Features:
- Deep penetration to seal pores
- Pre-metered unit eliminates mixing errors
- Fast drying for quick application
Pros
- Easy roller or spray application
- Bonds to slick or frozen substrates
Cons
- Not for extremely porous or damp concrete
- Shorter shelf life after opening
Bulldog Premium Universal Primer
Best styrene-acrylic primer
This water styrene-acrylic copolymer primer consolidates porous or dusting concrete for better adhesion. It has very low odor.
Key Features:
- Deep penetration into weak concrete
- Binds loose substrate particles
- Excellent basecoat for multiple floor systems
Pros
- Very low odor
- Fast recoat time
- Single component simplicity
Cons
- Not for extremely damp substrates
- Lesser moisture protection than epoxies
Kilz Klear Multi-Surface Uniformer and Sealer
Best low odor oil based option
This alkyd-resin sealer penetrates with superlow odor compared to typical oil based primers. It provides excellent adhesion to multiple surfaces.
Key Features:
- Exceptional vapor blocking
- Bonds without scuff sanding
- Mold and mildew resistant film
Pros
- Forms superior vapor barrier
- Binds glossy surfaces easily
Cons
- Longer drying time than water-based
- Not as penetrating as epoxy
Applying Primer Prior to Self Leveler
Once you have selected an appropriate primer for your substrate and conditions, proper application is key:
Surface Preparation
The substrate must be cleaned of all loose particles, dirt, grease, and other bond breakers. Cracks and divots wider than 1/8” should be prefilled.
Mixing
For 2-component epoxy primers, properly mix resin and hardener as directed. Pre-blended primers just need minor stirring.
Application Method
Use a short nap or smooth roller, following spread rate guidelines. Allow primer to fully dry prior to pouring self leveler.
Re-Priming
For very porous substrates, a second coat may be required to achieve proper penetration and sealing. This prevents substrate suction from starving the leveler mix.
Choosing primer by substrate
Porous, dusty concrete: an acrylic primer, diluted as the data sheet directs for the first coat, then a second coat at full strength once the first has dried. If the first coat disappears completely into the surface with no film left, the substrate is still thirsty and the second coat is not optional. Smooth or power floated concrete: the surface has little for a primer to grip, so use a primer the manufacturer rates for non porous substrates, and mechanically prepare the surface first if the data sheet calls for it. Calcium sulphate (anhydrite) screed: use only a primer the manufacturer confirms for calcium sulphate, and check whether laitance must be removed first, because a primer laid over laitance bonds to a weak skin that later lifts. Timber, plywood and OSB: use a primer rated for timber substrates and check the leveller is rated for timber too, because a primer cannot make an unsuitable leveller work. Damp or moisture risk slabs: an epoxy primer rated as a moisture suppressant is the usual answer, but only within the relative humidity limit on its data sheet, and only after a moisture test, not as a guess.
How much primer to buy
Work it out from the data sheet, not from a rule of thumb, because coverage rates differ between products. Divide the floor area in square metres by the coverage rate in square metres per litre printed on the data sheet, and round up to whole containers. Then allow a second coat over the same area if the substrate is porous, because the first coat on thirsty concrete is mostly absorbed and does not count as the bonding film. Buying one container short and stretching the last coat thin is a common cause of patchy bonding, so order the second coat into the quantity from the start.
Priming mistakes that cause failures
Pouring the leveller before the primer has cured to the time on the data sheet traps air and weakens the bond, and pouring long after the primer’s open time has passed can be just as bad, because a fully cured film may need a fresh coat to bond. Skipping the second coat on porous concrete leaves dry patches that suck water out of the leveller, so it cures unevenly and can crater or debond. Using a general purpose primer on calcium sulphate screed without checking compatibility risks the whole layer lifting with the laitance. And treating any primer as a moisture barrier without a moisture test leaves the real problem, damp in the slab, to attack the finished floor from below.
Conclusion
Using the right self leveling primer for your specific substrate and conditions is important to prevent flooring failures like poor adhesion, moisture issues, and uneven drying. Be sure to evaluate epoxy content, resin type, penetrating abilities and moisture protection to find the best primer for your project. With sound preparation, you can achieve excellent bonds and effective moisture barriers.
Frequently Asked Questions
Can self leveling compound be applied directly to concrete without primer?
It’s not recommended as it risks poor adhesion, moisture issues under flooring, and variable cure times. Use a properly formulated primer.
What temperature range is best for primer application?
Standard primers require a temperature range of 50-90°F during application and drying for proper film formation.
How soon can self leveler be poured after priming?
Allow acrylic and urethane primers to dry 1-2 hours before application. Epoxy primers require 6-24 hours before pouring self leveler.
Can primers be applied to damp concrete substrates?
It depends on the type selected. Check manufacturers guidelines, as some can be applied to damp concrete up to 100% RH.
How do you know if concrete needs a second coat of primer?
If the first coat fully penetrates without leaving a film, the substrate is too porous. Re-prime to build an adhesive film layer.