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How to Harden Polyurethane

Gregg Miller

Other than adding a hardening agent, which can negatively affect the sheen and usability of polyurethane, employing the right techniques and materials are the best ways to ensure that the finish reaches its maximum strength and durability. There are many factors involved in the the hardening process of polyurethane.

Stairs can take a lot of abuse, but a fully hardened coat of polyurethane is up to the challenge.

Some of these include humidity, proper mixing techniques and adequate surface preparation. If these industry-accepted practices are closely followed, the polyurethane will harden to an appropriate strength and perform to the expectations of the manufacturer and consumer.

  1. Before application, use a drill-mounted paddle wheel to thoroughly mix the polyurethane. Polyurethane cures through a chemical process once it is exposed to air, and if all the components are not properly mixed, it can retard or even defeat the curing process. This can leave the polyurethane soft and possibly keep it from drying at all.

  2. Properly prepare the project. Polyurethane is designed to be applied in multiple layers or stages. Between each one, the necessary preparation has to occur or the successive layers may fail. Depending on the type of project, proper surface preparation may include sanding or tacking with chemicals. Follow the manufacturer's guidelines to meet this requirement and the polyurethane will harden properly.

  3. Provide adequate ventilation. Because polyurethane depends on ambient conditions to harden, make sure that you have proper ventilation in the area where it is being used. Inadequate airflow can retard hardening and slow down your overall project while waiting for the polyurethane to cure.

  4. Allow each coat time to fully harden before applying the next layer. A premature second application can prevent the first from drying, because the thinners don't have time to dissipate into the air. This will leave a soft finished product that may never harden. In extreme circumstances, you may even have to start the process all over again.

  5. Warning

    Ventilation is essential to the hardening process, but don't let air move too freely across your work surface. It can carry debris that can settle into wet polyurethane.