Q.: We have a problem with too many bug holes on wall surfaces formed with rubber-lined wall forms. Are there ways to achieve the same smooth surface we can get with bare medium-density-overlaid plywood wall forms?

A.: We're not sure we know what you mean by "rubber-lined" wall forms. Are the forms lined with rubber sheets or mats as described in Formwork for Concrete, published by the American Concrete Institute? Or are you referring to an elastomeric form liner?

In any case, rubber or elastomeric liners are more impervious than MDO plywood and could produce more bug holes if all other conditions were equal. Because it's more pervious, an MDO form absorbs water and permits the passage of air and water from the concrete, thus reducing the size and number of bug holes.

Form-surface absorbency also affects the thickness of the layer of form-release agent. A rubber or elastomeric surface, which is impermeable, won't absorb any form-release agent; an MDO surface, which is semi-pervious, will absorb some. So if you apply the same amount of form-release agent to both surfaces, a thicker coating will result on the rubber surface. Thick layers of release agent increase bug hole size and number. Ask the rubber-liner manufacturers to recommend a release-agent type and application rate.

It's also possible that the rubber surface is rougher than the MDO surface, making it easier for bubbles to cling to the surface. And if the rubber liner is installed to impart texture or pattern to the concrete, as is often the case, the surface area of the liner may be two to three times that of a flat form, requiring significantly more vibration to achieve a satisfactory surface.