"Without it (bonding primer), there's nothing for the coating to grab onto," says Pachade. If the concrete substrate is not primed, "You'll see pinholes because the concrete is breathing."
Bonding primers serve two purposes when applied to concrete before topcoating, according to Victor Pachade, national sales manager for Smart Surface Technology, manufacturer of Colormaker products. “They seal the concrete surface and develop a bond line for a coating or topping to bond to the substrate.”
Concrete isn’t usually a good substrate for holding coatings because of its high porosity and alkalinity. “Without it (bonding primer), there’s nothing for the coating to grab onto,” says Pachade. If the concrete substrate is not primed, “You’ll see pinholes because the concrete is breathing.”
You will have even more problems if you’re topcoating over old concrete. The substrate might well be dry and crumbly, causing the surface to flake and take the new coat with it.
The solution is a layer of bonding primer that provides benefits during both the liquid phase and the solid phase of the application. In addition to sealing a porous substrate and adhering to the topping, primers increase the wettability of the topcoat, making it easier to spread.
“This increases the working time,” says Bob Harris of the Decorative Concrete Institute in Temple, Ga.
At the same time, it decreases the risk of a fisheye effect brought on by partial adherence of the topcoat, says Pachade. “Without it, your topcoat will come up in three months.”
To Harris, it’s easy to determine when to use a bonding primer: when the manufacturer specifies it. It’s also necessary to use a bonding agent when the surface will be subjected to extreme temperature conditions or in conjunction with a self-leveling or semi-self-leveling surface, he says.
Also, use a bonding primer whenever topcoating concrete, says Pachade.
You’ll find bonding primers in acrylic, epoxy and polyurethane composition. Often, acrylic primers are used before applying a urethane topcoat. “Urethanes don’t like the alkalinity of the concrete, so they repel it,” says Pachade.
It’s also wise to go with an acrylic primer when applying it to an exterior surface. “Most epoxies tend to yellow outdoors, and urethanes can be very slippery,” says Pachade.
Regardless of which primer you’re using, be sure to thoroughly wash your surface before applying.
If your primer is re-emulsifiable, it must be completely dry to the touch before applying your topcoat. If it’s not re-emulsifiable, the topcoat is usually applied while your primer is still wet.
It generally takes two coats to do the job, and you’ll get 200 square feet to 300 square feet of coverage per gallon. “You can use a sprayer with an acrylic primer, but roll or brush with a urethane or epoxy,” says Pachade.
One final piece of advice from Harris: All bonding primers are not the same. “Be sure to take that into consideration as you prepare your estimate, because the drying time will vary.”