Paint Quality Institute

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Frequently Asked Questions
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PARAFFIN

Question
We need information on wax bleed. Can you explain what it is, and how to address it if it occurs?

Answer
Wax bleed is a phenomenon that can occur with exterior hardboard siding as it weathers. Hardboard siding is made by pressing wood fibers under heat, with several per cent of paraffin waxes included, to enhance water resistance, and minimize water absorption, which could otherwise lead to swelling and warping of the siding. The wax may migrate to the surface and accumulate there. It comes through a factory primer if left unpainted, or through some paints. (Fortunately, manufacturers of hardboard siding have improved the quality, and thus wax bleed resistance, of factory primers used.) It then tends to pick up dirt, and mildew can grow, too.

We have conducted exposure tests on repaint of wax bleed. Acrylic house paints tend to have adequate adhesion to the surfaces we have tested, despite the hydrophobic nature of the wax. However, we have also found the resistance to wax bleed-through itself is related to an extent to the level of pigmentation. Flat paints resist wax bleed less that satin paints, for example. We have not seen wax bleed when a quality acrylic latex exterior primer is first applied.

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