Expanded polypropylene (EPP) foam is a synthetic material often used in the construction of model aeroplanes. While it is desirable for its flexibility and impact-resistant construction, EPP is also porous and heat-intolerant making it difficult to paint. Even when properly applied, paint on EPP is not durable and may chip and flake as the plane is used.
When EPP is cut or shaped, air pockets within the foam are exposed, making the surface uneven. Modifying the surface of the EPP will ease the paint application process. Apply vinyl spackle or a watered-down polyurethane glue to the surface of the plane, then sand to smooth bumps. A plastic shrinkwrap may then be applied to completely cover the aircraft. This provides a smooth, even surface for painting. Both the filler and the following paint application add weight to the plane, so consider this additional weight during calculations.
Even pretreated EPP is difficult to paint. Avoid using brushes and rollers to apply the base coat as they can ding the surface of the model. Water-based spray paint is ideal for use on EPP. Examples of suitable paint include Maston ColorMix H2O and RustOleum water-based polyurethane. Once you have applied two to three base coats, you may use acrylic craft paint to add details with a brush. Coat the entire body of the plane with a clear sealer.
Aesthetic elements such as logos, lining and colour schemes may be original or derived from an actual aircraft. When choosing a design, consider the complexity of the pattern. For example, modern stealth aircrafts feature few if any regional markers and range from matt silver to black. This makes reproductions recognisable with a plain base coat. While the F-16 Thunderbirds used by the U.S. Air Force for air demonstrations have a more complex design with striped winglets and Native American motif. Such a pattern would require stencilling and hand-painted detail.
Minimise the number of colours in your plane design to make painting easier. Apply the lightest colour first, and then follow with darker sections. Tape off areas for detailing with painter's tape or similar light-tack tape. Always practice your design technique on a piece of scrap foam before trying it on the actual model. Work with a moistened brush, as wet bristles are less likely to scrap off the underlying base coat.