Polyurethane foam is frequently used as a modelling material by hobbyists and wargaming enthusiasts, as well as in a wide variety of other applications such as furniture cushions and insulation. Polyurethane foam should only be painted with spray paint if it is of the closed-cell type. Open-cell polyurethane foams tend to melt when subjected to aerosols.
Take a very close look at the surface of the polyurethane for any irregularities. These can include small holes left by air pockets, and small amounts of extra polyurethane called flash. Fixing these problems after the foam has been painted is certainly possible, but it will likely mean that the foam must be repainted. Carefully cut off any flash using a craft knife and sand until smooth. Holes left by air pockets can be filled with a sandable putty such as Bondo.
Sealing and Priming
There are two types of polyurethane foam: open-cell and closed-cell. Open-cell foam is softer, because the material is porous and allows air to penetrate. Closed-cell foam is non-porous and is considerably stronger. You must seal open-celled foam before painting it. You can do this by mixing together white glue and water and brushing it onto the foam. Thoroughly clean the brush immediately afterward. Closed-cell foam can be painted without sealing. Apply an acetone-based primer directly to the foam.
Allow the primer to dry thoroughly before painting. You can use almost any type of paint on polyurethane. This includes both acrylic and oil-based paints, as well as spray paint. Apply a base coat and allow it to dry. Apply a second and even a third coat until you have achieved the effect you want. Several thin coats are better than one thick coat, as it will help to preserve details. Paint the lower areas of the foam first. It is much easier for the brush to catch a raised area and miss a lower one rather than the opposite. Painting the raised areas last will help to cover up any mistakes.
Applying one or more washes can really help to bring out details. Make a wash by mixing paint with water. The proportions depend on the colour of paint being used. You can make lighter coloured washes with less water than darker ones. A black wash can be made up of mostly water with a few drops of black paint added. A black wash applied at the final stage can help to create a shadowy effect.