Why Won't Oil-Based Exterior Paint Dry in a Bathroom?

Oil-Based Paint Properties

• Oil-based paint is formulated using binders, pigments, additives and alkyd resins. It is slow to dry compared with water-based latex paints, taking up to eight hours to dry to the touch compared to under one hour for latex paint. Thinning and cleanup requires mineral spirits. It dries to a harder and less flexible finish than latex. The optimum drying time for oil-based paint is about 70 degrees Fahrenheit at 50 percent or lower humidity.

Drying Factors

• A number of external factors influence paint drying time. Temperatures lower than 60 degrees Fahrenheit and humidity of 80 percent or higher will significantly slow drying time. If the humidity level remains greater than 90 percent, oil-based paint can take an extremely long time to dry. Paint will also dry slower in very still air. Air flow helps the paint dry by evaporating the liquids from the paint film, so there should ideally be a slight breeze.

Substrate and Application

• Both the underlying surface and how thickly the paint is applied affect drying time. If the underlying surface is hard and shiny, paint will take longer to dry than when painted on to a more porous surface. If oil-based paint was applied to damp walls, it will take a very long time to dry and will eventually bubble and peel. A very heavy film of paint will also dry more slowly. If the paint is a very deep color, be aware that some pigments can also make the paint dry slower. Call the paint store and ask if the pigments used could be slowing the drying time.

Using Exterior Paint Inside

• Exterior paint has more additives than interior paint. Chemicals that protect the paint from sunlight, temperature extremes and mildew can retard drying. Volatile organic compounds (VOCs) are potentially harmful chemical compounds that are released into the air as paint dries. Oil-based exterior paint contains higher VOC levels than interior or latex paints.

Prevention/Solution

• To speed up oil-based paint dry time in a bathroom, open the windows or use a fan. If it is very humid, putting a dehumidifier in the bathroom will help a great deal. Make sure it's at least 70 degrees.

  To speed drying time with oil-based paints in future projects, consider adding Japan drier to the paint. When used as directed, Japan drier can cut dry time by about 50 percent.