Installation of CRU Coating Systems

Chemical Resistant Urethane Coatings are widely popular, in particular for warehouse and hangar floors. The typical specification is for an epoxy primer followed by two coats of a two-component, chemical resistant urethane topcoat. There are variations that add an epoxy fill/body coat to hide substrate profile created during surface preparation.

Please note: without a body/fill coat these systems are typically less than 12 mils thick. This is at or less than the thickness of a business card. Every substrate imperfection will be visible as will roller fuzz, construction dust, etc. This system should only be used for foot and soft-wheeled traffic. A hand tool dropped from a few feet, if on edge, will chip through to the concrete. Owner expectations, based on installed cost, are often far in excess of what can be achieved in aesthetics and performance.

In an effort to assist in the installation process please review the items below prior to starting a project.


Surface Preparation: Vacuum, steel shot, blasting is by far the preferred method of surface prep. A profile of CSP 1 or 3 is recommended as detailed by ICRI in Guideline No. 03732. If this is not familiar, the profile should appear similar to medium to fine grit sand paper. Shot blasting will leave a pattern. To hide this profile it will require a minimum of 15 mils of finished coating thickness. For best results use a fill coat of epoxy to hide the profile prior to applying the topcoat. Any substrate imperfection, cracks, divots, spalls, pop-outs, over lap of blast pattern, etc. will show through. All imperfections should be pre-filled prior to coating.

Acid etching is often used for these Systems. It will not leave a pattern, if done properly, which can reflect through the coating. With this said, acid etching is not preferred. It can be inconsistent in profile, introduces water into a porous substrate, and a byproduct of the acid base (concrete) reaction is the formation of salts. If these salts are not removed with the rinse water they will dry on the surface and inhibit long-term bond. If acid etching is the only viable option, refer to ASTM D4260 (good) or ICRI Guideline 03732 (best) for details. These are available through your General Polymers Representative or Technical Service.

Joints and Cracks: Honor all expansion/isolation joints (do not coat). Fill with the appropriate joint material, flexible or semi-rigid, after installation of the coating. Saw cuts, cracks or control joints can be filled and coated over in a temperature controlled environment. For proper procedures/techniques and products visit our website or contact your General Polymers Representative.

Odor: The vast majority of CRU's (Chemical Resistant Urethanes) are solvent-based. Odor can be a problem both during and for several hours after adequate ventilation is a requirement. Most formulations are also flammable. Safe handling and storage are important; refer to the Technical and Material Safety Data Sheets of the specific products.

Environmental Conditions

Temperature: These products should not be applied at less than 50 F air, product or substrate temperatures. Working time, cure time, viscosity (mixing & flow), air release, surface tension and color stability will all be affected by temperature. These products work best at 70-85 degrees F in a combination of surface, product and air temperature. Outside of this range care will need to be taken to insure success. Know your temperatures before you start, buy an infrared thermometer for each crew. A product temperature of 70 is ideal, but if placed on a 45-degree substrate it will not flow, release air or cure as hoped.

Humidity/Moisture: Urethanes are the reaction product of a Polyol (Part A) and an Isocyanate (Part B). Water is also a Polyol. The presence of moisture will accelerate the cure of a urethane but most problematic is the formation of foam/bubbles in the coating. The point being: make sure the surface and all equipment are dry and if possible avoid installing during rain or high humidity conditions.

Dew Point: Dew point is the temperature at which moisture is converted from a gas to a liquid. Moisture will condense on a cool surface if the temperature is at or below the dew point. Yes, this can occur on an interior slab if the building temperature/environment is not controlled. It is difficult to impossible to see moisture on an absorbent surface like concrete. The moisture present can be enough to inhibit bond and bubble. To eliminate this potential check the dew point. Do not coat if you are within 5 degrees of the Dew Point. If this is not possible leave a fan running overnight in the room to be coated. Have you ever seen dew on a windy morning?

Moisture Vapor Transmission: Coatings are the most likely impermeable resinous floor system to fail when excessive Vapor Transmission rates are present. Use Calcium Chloride Test Kits to verify prior to coating. If readings are above the acceptable limits contact General Polymers Technical Service Department. If you are not familiar with Calcium Chloride Test Kits, how to use them and what they mean contact your General Polymers Representative or the Technical Service Department.

Air Movement: Avoid high airflow over the coating surface from ventilation or wind. This can cause premature solvent evaporation, which will impact flow/leveling, air release, etc.


Mixing: Match your drill speed and paddle to the volume to be mixed. The spiral type paddle works very well with coatings. Avoid a vortex due to paddle size or drill speed; this will whip air into the coating. Do not "pump" the paddle up and down. Quantities less than a gallon can be mixed by hand. Use only new or very clean containers, the solvent in the material can dissolve container residue. All components should be pre-mixed, especially the Part A, which contains the pigment. Do not use a paddle with A/B residue to pre-mix a new container. Do not use a paddle with epoxy primer residue to mix a urethane. The Part B (amine) of an uncured epoxy will react with the Part B (isocyanate) of a urethane.

Mix no more than can be applied in 30 minutes. Do not let mixed material sit in volume. Once mixed, dump the entire quantity on the floor. Product left in the bucket for several minutes will be at a different stage of cure than material first placed. This can affect flow, texture, air release and color stability. The higher the viscosity of the material being mixed the longer you mix.

Priming: Yes, you should. Good primers penetrate the substrate and provide a good bond site for the next application. They also reduce concrete out-gassing and provide some film build to reduce surface profile. Match your primer to the coating system. Most manufacturers make several primers for different conditions: low temperature, oily substrates, solvent or 100% solids, blush resistant, etc. A solvent-based epoxy primer works well as it is low in viscosity and is less likely to leave roller or squeegee lines. Check with Technical Service for a recommendation if in doubt. As a general rule use the fastest curing, low viscosity primer available. This combination works best to reduce out-gassing from the substrate.

Application: Solvent-based urethanes of this type should not be applied in thick films. Even 100% solids CRU's should not be applied in thick films, typically 6 mils or less. A byproduct of the Isocyanate/Polyol reaction is carbon dioxide, which is a gas at room temperature. In films applied too thick the surface (interface with moisture) will cure first, and as the material below reacts, CO2 will be trapped causing bubbles. You essentially have created urethane foam. This will appear as small, trapped blisters, or a dull and soft film. The other issue associated with thick films is the potential for solvent entrapment, which also will create bubbles. As a general rule these products will produce the best results when placed at 4-6 mils (wet). At applications less than 4 mils roller texture will be increased as there is not enough film to level.

Apply the material over a tack free primer via flat squeegee or spring steel blade and back roll with a high quality short nap (3/16 or ") roller. A squeegee distributes the material evenly and is the most efficient in terms of time. It also insures the entire mix is curing at the same rate. If you cannot use a squeegee, pour the entire mix in a ribbon and roll in and out of the ribbon. Keep the roller fully wet out (saturated). As a roller empties it fills with air, which will now be put into the coating. Dip and rolling out of a bucket may work occasionally but it is not a good practice. Purchase spiked shoes and learn to use a squeegee.

For the best results follow the initial rolling process with an 18" roller at a right angle, after the material has sat for 10-20 minutes. This will reduce roller lines and even out any color change from mix to mix.

At the application thickness discussed any roller fuzz, debris or construction dust that settles will be visible in the cured film. The film is not thick enough to hide any of this debris. Tape all roller covers and pull off any loose nap prior to application. Pre-wet the roller with solvent (MEK) and spin- off the excess to reduce drag prior to rolling the final topcoat.

Recoat Window: Follow the guidelines listed on the Technical Data Sheet for the primer, fill resin or topcoat as the case may be. Recoat windows will be impacted by temperature. A recoat window of 24 hours, at 70F, will be reduced by several hours if installation temperatures are well above 80F. If in doubt, or you are at or beyond the recoat window, sand and tack wipe prior to coating.

Trouble Shooting

Fish Eyes: They occur due to a difference in surface tension between the coating and the substrate. This can be the result of a contaminant (oil, grease, dust, sealers, etc.), amine blush, primer outside the recoat window, moisture, etc. In most cases good surface preparation will solve the problem. "Fish eyes" will typically occur shortly after application, when noticed, stop coating they will not fix themselves! To verify if the problem is substrate or product related, mix a small amount of the material and apply to a sealed surface outside of the project environment. If this does not "fish eye" you know the problem is on the substrate. If this also "fish eyes", do not continue, call Technical Service. General Polymers and Sherwin Williams do supply "fish eye" reducers. Urethanes are more likely to have this occur than are epoxies, be prepared and have a reducer on hand. Contact your GP Representative for supply.

Air Bubbles: These can result from a variety of factors. Typically, if the problem is in the material, bubbles will occur uniformly and within 30 minutes of application. This can be the result of the product, temperature, mixing or application technique. When coating an excess broadcast floor the bubbles are often the result of trapped air in the texture that expands as the day heats up. Make sure you are not whipping air in during mixing. Keep the roller fully wet out. Sometimes you can break the bubbles by re-rolling after the material has set for 20-30 minutes. This is temperature dependent as you can also change the texture if the material is too far along in cure. A porcupine (spiked) roller can also be helpful to break air bubbles if they are fairly large and not extensive. This must be done soon after placement. Do not use if the material has any tack or you can leave small dots of a different color shade.

Check substrate and product temperature. The thicker the film and lower the temperature the more difficult it is for the resin to release air. Have an air release additive on hand for all coating applications, as a precaution. Contact General Polymers for a recommendation.

Substrate out-gassing is difficult to predict or anticipate. Priming or coating late in the day, as the slab temperature is falling, is a good practice. These bubbles occur late in the cure, often after the contractor has gone. The product is no longer fluid and will not flow back to close the hole. They appear as small craters, with raised edges. To repair they must be sanded smooth and the hole filled prior to coating, or they will reappear.

Amine Blush: This phenomenon can occur if using an epoxy primer. The name refers to the curing agent/hardener, which is an amine. It can and will react with moisture and carbon dioxide in the air to form the blush. Dependent upon the formulation it is most likely to occur at low temperatures or high humidity and is worse when in combination. The blush should be visible as a film or haze on the surface that reduces gloss. It is noticeable by touch. It can be removed by a warm water detergent scrub, solvent or mechanical abrasion (sanding). A blush can also be an indication of improper mix ratio or an incomplete mixing. The Part B is less dense than the Part A and will come to the surface if in excess or not properly mixed. An amine blush will inhibit bond and or create "fish eyes".

Color Change/Pigment Float: Use only one batch of topcoat for a project. If multiple batches are used, box the material. If a pre-mixed container of Part A has sat for more than one hour, mix again prior to use.

As discussed earlier a urethane coating is a chemical reaction. It typically takes hours to reach completion. The pigment(s) are solid particles in suspension within a film that is cross-linking. This is why you can touch the product well into cure and the color can change, typically lighter, as you disrupt the process. Plan your project to minimize the time between mix-to-mix tie-ins. This is formulation and temperature dependent but a good rule is; try not to go beyond 20 minutes. Do not roll into a partially cured edge. Use joints or other natural breaks to minimize the time between mixes. If you cut-in too far out in front you will need to re-roll over this material to avoid a shade differential. This potential for color change is a primary reason to do a second back roll, at right angles, to blend all mix-to-mix tie-ins.

Special color requests are more likely to have a pigment flotation issue than standard colors. It can be more pronounced in dark/deep blues, browns and greens. This is a one- time formula with which we have no history or experience.

Specialty Tools

  • Spiked shoes - Those back rolling will need to walk in wet material.
  • Porcupine rollers- To remove air bubbles, while the material is still wet
  • Mil gauges- To insure thickness and coverage rates
  • Infrared Thermometer- Do not leave home without it
  • Dew Point/Humidity Meter (Psychrometer)
  • Air Release Additive/Fish Eye Reducer
  • Adhesive roller covers are great to back roll with but can be hard to find. Some Home Depot and Sherwin-Williams stores do carry them. They shed almost no roller fuzz/lint. They should only be used to back roll as they hold little material and must be kept full to avoid whipping air into the coating.