Open Time


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This article was previously published in The Faux Finisher Magazine, Summer 2009.

It explains the difference between Open Time and Working Time and offers a test for painters to determine how much working time the glaze they're using actually has.

How much Open Time does your Glaze have?

“How much open time does your glaze have?” is probably the question I hear the most when I attend tradeshows or answer technical phone calls concerning decorative painting materials. Technically, the question refers to the length of time glaze will move over a surface before forming a film. The terms “open time” and “working time” are used interchangeably in the decorative painting industry; however, this is technically inaccurate.

What painters really mean to ask is, “how much working time does the glaze have?” Working time refers to the amount of time that the glaze can be manipulated before it begins to “set up.” At this point, while the film may still appear to be wet, it is immobile and can no longer be worked. Open time refers to the total amount of time the glaze is not dry. Most painters know what it’s like to experience the end of working time of a glazed surface. At that point, all that's needed is one more blending touch and the whole section lifts. The surface is still wet and while it may be “open,” it cannot be worked without damaging the finish.

A water based glaze will not perform exactly the same in all environments, over any surface and with any combination of colorants. For waterborne products, there is no single answer to the question “how much working time does your glaze have?”

The working and open times of a waterborne glaze are determined by many factors, including the glaze formulation and its raw material ingredients, the absorbency of the surface the glaze is applied to, the tools the glaze is manipulated with, the technique, thickness of the application and skill level of the painter applying the glaze and most importantly, the environmental conditions including air temperature, relative humidity, and amount of air flow. Most of the factors in determining working time are completely beyond the control of the glaze manufacturer. Instead, to a degree, many of these factors lie with the painters and their ability to execute the chosen finish.

The raw materials which have the most dramatic effect on working time and open time are glycols and “extenders.” Some painters will rely on these materials to provide more working time than has been formulated into the product, but these materials fall into the category of volatile organic compounds or VOCs. VOCs are regulated by the local, state and federal governments because of the role they play in the formation of ground level smog. Regulations vary by location, and it is the painter’s responsibility to stay up on the local laws wherever he or she is working to make sure that the mixtures they are using are compliant with local regulations. By law, any container larger than one quart is required by law to have the VOC content printed on the label. In the US, the VOCs are reported as grams of VOCs per liter of product, less water. This means that the water content of the product is not considered to be part of the volume of the product, and thus, results in a stricter limit. If you don’t see this information on the label, you can call the manufacturer and ask. Keep in mind that other countries calculate VOCs differently (or do not limit them at all), so not all products made outside of the US may be suitable for use in your location.

Regardless of the terminology, just what are those times on glaze labels and brochures actually referring to? Without knowing how a particular manufacturer tests for open time, it is difficult to say. A commonly used “standard” method in the paint industry is really intended to measure drying time. In this test a non-absorbent test card is coated with a uniform film of the material. The card is then placed in a device that causes a stylus to slowly “draw” a circle in the wet film. When the stylus no longer leaves a mark in the film, the time is recorded as the “drying” or “open” time. Of course, while the test is running, the wet film is not being bagged, brushed, and manipulated – all of which tend to speed up the evaporation of water from the wet film.

One of the ways we measure the open time in the GOLDEN Laboratory is by coating a non-absorbent test card with the product and then the technician will use his or her finger at timed intervals to pull through the glaze, observing how long this can be done before the glaze begins to dry, resist smearing cleanly and leaves a dried edge. At the first sign of an edge, the working time has stopped. However, if the glaze can continue to be pulled, the material would still be considered “open.”

While providing more information than a simple drying test, even this method is not a reliable indication of what might happen on a jobsite. So, the most that can be expected of a manufacturer is to produce a formula that, in a controlled set of conditions, consistently delivers the balance of properties they believe their customers desire. The reality is, of course, that the environmental conditions on any particular jobsite are likely to be different than those in the manufacturing environment. Recognizing this, the savvy painter can determine the working time he is likely to encounter on the job by creating a test similar to the following:

  • Try to recreate the environmental conditions that will be painted in as closely as possible including: room temperature, humidity levels and airflow.
  • Recreate the surface to be painted as closely as possible. For example, if the surface to be glazed is wood, get the same kind of wood and prepare it the same as the site to be painted including primer and base coat. If the surface will be drywall, then get a piece of drywall. Glazing on polystyrene sample boards is common practice at tradeshows and in classroom setting, but is not representative of most people’s homes!
  • Mix up the glaze and apply it to a section of the sample. Wait 15 minutes and then apply an equal amount of glaze next to it. Blend or tool the two glazed sections together. If laplines appear, the glaze combination may not give the adequate working time needed. Wait another 15 minutes, apply more glaze and blend back into the previous sections. By repeating these steps, a painter can get a reliable indication of the working time under the given conditions of substrate and environment. Keep in mind the thickness of the glaze application can alter the results. A thicker application will slow down the water evaporation, whereas a thin coat will allow faster evaporation.

Earlier, we discussed how the use of glycols and extenders can provide additional working time. However, one downside to this approach is that these materials can also dramatically increase the overall drying time, resulting in a prolonged period during which the work remains sticky and fragile with poor “strapdown,” during which time no further work can be done. Alternatively, an understanding of how ambient conditions affect the performance of a glaze, allows painters to put this knowledge to use to influence the open time of a glaze without causing excessive wait times before work can resume.

If more working time is desired, increase the humidity level in the room by turning on a humidifier and closing the room up; decrease air flow, and decrease the air temperature. If the glaze needs to dry faster, do the opposite and decrease the humidity, increase air flow and increase the air temperature.

Understanding the differences between open time and working time of a waterbased glaze is an important aspect of knowing the materials of a decorative painter. The more a painter understands the materials, the more control one will have in applying them. Happy Glazing!

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