White wrapping paper from WalMart provides a strong and smooth surface for foam parts.  Only one side of the paper has glue applied to it and the other side is coated with a paint of your choosing.

Uses ALEENES 70-2, Flexible Stretchable Fabric Glue.  Available from WalMart in Crafts Section.  This glue provides a much superior bond to foam than the white glue or water based polyurethane previously used.  In fact, it can also be heated after drying with an iron to provide an even stronger bond, but I think you will find the regular bond is very strong.   The smooth surface of the white paper provides a better looking painted surface in comparison to the rough brown paper.

Another benefit of the Aleene's glue is that it doesn't shrink the paper like the other glues.   One side of an elevator or rudder can be covered without the part bending into a U shape.

1.  Cut out a piece of white paper large enough to extend over the sides of the piece you want covered.   
     (On a wing, this can be wrapped around the leading edge.)
2.  Squirt a little Aleene's on the foam and spread it evenly across the surface.  I use my finger.
3.  Lay the paper on the foam and press firmly together.   You can use a cloth under your fingers to rub the paper down tight against the foam.
4.  Let dry overnight.
5.  Use an exacto knife to cut away the excess paper.
6.  It is now ready to be painted.

AREAS to apply paper.
1.  Fuselage sides at least to several inches past the rear of the wing.
2.  Front 1/3 of wing on both sides.
3.  Motor mount areas.
4.  Areas around control horn  mounts.
5.  Any other area subject to abuse or high stresses.