For Producers

Form Release Agents for Urethane Precast Concrete Molds

Most release agent advice is written for steel forms. Here is what carries over to a textured urethane fence mold, what does not, and how to apply it so the face releases clean.

21 October 20265 min readElastoquip

For a urethane precast concrete mold, use a chemically active (reactive) form release agent that both your mold maker and the release agent maker approve for the urethane face, and apply it as a thin, even coat on a clean mold with no puddles left in the texture. The NPCA Quality Control Manual requires release agent to be applied according to the manufacturer's instructions after the form is cleaned, and requires any puddle to be removed before casting. On a deep stone or woodgrain face, getting the amount right matters more than the brand you buy.

Should you use a barrier or a reactive release agent?

A reactive release agent is the better choice for a decorative fence panel because it leaves fewer surface defects. The American Concrete Institute (ACI) describes two main types: barrier agents such as diesel oil, wax and silicone, which sit between form and concrete, and chemically active agents, whose fatty acids react with calcium in the cement paste to form a soap that stops the concrete bonding. ACI says barrier agents are not recommended for architectural concrete because they can cause stains, surface air voids and stripping problems in very hot or very cold weather.

The National Precast Concrete Association (NPCA) gives the reason in its Tech Notes on release agents. A heavy barrier coat tends to trap free air along vertical sidewalls, while the soap formed by a reactive agent lets that air rise out. The Precast/Prestressed Concrete Institute's plant quality talk on bugholes says the same thing in one line: use a reactive release agent, not a barrier type.

Does steel-form advice apply to a urethane mold?

Only part of steel-form release advice applies to a urethane mold. NPCA explains that the fatty acids in reactive agents have a natural affinity for metal and react with it to form a protective coating, a process called seasoning, and NPCA calls seasoning a very basic requirement for steel forms. A urethane face is not metal, so seasoning routines written for steel tell you nothing about how a given product behaves on your mold.

The metal parts of a fence mold still matter. NPCA cautions that fatty acids react with brass, aluminum, mild steel and some petroleum-based products used for blockouts and embedded items, and recommends stainless steel, nickel or plastic sprayers. Test any insert or blockout material before you change release products. Transportation agencies take the same compatibility view for textured form liners: the Iowa Department of Transportation requires release agents that are compatible with the liner material and non-staining, applied per the liner manufacturer's recommendations. Ask your mold supplier which release chemistries suit their urethane before you buy a drum.

How much release agent does a urethane mold need?

A urethane mold needs far less release agent than most crews apply. NPCA's Tech Notes put the effective coat at only about 0.005 inches thick and give the rule of thumb plainly: less is better. Release agent also thickens in a cold yard and thins in a hot one, so the same sprayer setting lays down a different coat in January than in August.

NPCA's guidance on application methods is practical:

• Spraying is the most common method. Keep the wand moving and use broad flat spray tips for the thinnest, most even coat.
• As the yard gets colder, raise spray pressure and use a smaller orifice. As it warms, lower the pressure and use a larger nozzle.
• Wiping with a sponge or rag gives the thinnest coat of all but takes the most labor.
• Brushing, swabbing and dipping tend to leave a coat thicker than necessary.

Apply release before the reinforcement goes in. NPCA requires reinforcement and embedded items to be free of release agent, because steel or an insert coated with it may fail to bond to the concrete.

What goes wrong in deep texture?

Release agent pools in the low points of a textured face, and every pool becomes a surface defect. NPCA's architectural repair guide explains that a pool or droplet keeps concrete out of that space, so a bughole is left where it sat once the form is stripped. PCI adds that too much form oil causes very small bugholes and dark staining. On a woodgrain or stone texture, the recesses that carry the detail are exactly where that excess collects.

Check the low points after spraying and wipe out anything that has gathered there before the pour. Our guide to wet cast concrete for decorative fence panels covers how vibration and mix interact with release to produce bugholes.

How should you clean between casts?

Clean the mold after every cast, ideally right after stripping. NPCA requires forms to be cleaned after each use, notes that waiting lets concrete bond more tenaciously, and says release agent coatings shall not be allowed to build up. Build-up matters on a textured face because it softens the detail that the panel is sold on, and PCI lists build-up and surface roughness among the causes of bugholes. Our article on how long polyurethane concrete molds last covers cleaning tools that protect the urethane.

What do VOC rules require?

Every form release agent sold in the United States must meet a federal VOC limit. The EPA's architectural coatings rule caps form release compounds at 450 grams of VOC per liter, or 3.8 pounds per gallon. ACI notes that some states set a stricter 250 g/L (2.2 lb/gal) limit, and that diesel oil, once a common release, is now prohibited because its emissions contribute to smog. If you ship panels or run a second yard in another state, check that state's limit as well as the federal one.

Can you switch release agents mid-job?

Avoid switching release agents during a job, because the panels you have already cast were finished on the old product. The Washington State Department of Transportation's standard specifications state that a change in form release agent will not be allowed on form liner finishes. If you do change products, cast a test panel first and compare it against your current production.

Write the choice into your QC routine. NPCA's pre-pour inspection checks release agent application on each form, with documented verification on at least one setup or 3% of each production run, and records kept for three years.

If you are setting up a fence line, our mold systems page lists Elastoquip's urethane gang, rail, post and cap molds, and you can ask our team about release agents for them before your first pour.