Using an inappropriate primer can cause paint failure. Though urethane paint is very durable, the surface to which it is applied largely determines which primer to use. However, for specialty applications like marine or automotive, there are additional considerations. As with all primers, proper surface preparation and weather conditions must be provided to ensure proper adhesion. With a proper primer, urethane paint can last for years.
Types
All-purpose primers are acceptable for most applications. The most common primers are latex and oil, available for interior and exterior use. Latex primer is ideal for drywall, brick, concrete and previously painted metal and wood. Oil primers work best on bare wood due to their wax- and tannin-blocking properties. Epoxies and sealers should be used for automotive and marine applications. Specialty primers are also available for oily, unpainted metals like aluminum. To enhance finished color, use a tinted primer.
Surface Preparation
Only prime clean, dry surfaces. Remove all surface imperfections such as rust, mold, dust or loose paint. Attempt to clean heavily rusted surfaces, even if a rust-inhibiting primer is used. For nonporous surfaces like metal or plastic, it may be necessary to sand with a fine-grit sandpaper, emery cloth or steel wool to ensure proper primer adhesion. Rinse the surface with water to remove debris from sanding.
Application
Depending on the brand of primer and the painting surface, it may be necessary to apply multiple coats of primer. For example, automotive applications can require seven or eight coats of primer and sealer to achieve the desired finish. Lightly sand between coats with an extra-fine-grit sandpaper or emery cloth. When outdoors, do not apply primer when raining, snowing or if the temperature is below 50 degrees Fahrenheit. Drying times increase in humid conditions. Do not apply another coat until the undercoat is fully dry and cured.
Warning
There are many primers that contain VOCs (volatile organic compounds) that can be hazardous to health. When using VOC primers, always ensure there is proper ventilation. Care should also be taken when preparing older surfaces. Paint made before 1978 may contain toxic quantities of lead. Always wear a respirator when using primer in spray form, regardless of VOC levels.