Here are some tips on one way to paint a foam base without worrying about spray equipment and professional painting skills. [We don't presume to offer advice to experienced painters - they should be able to adapt their methods without much difficulty.]
Apply paint in normal fashion, starting at one corner and only covering about 2 - 4 square feet at a time. Use enough paint so that you can be sure to cover all the contour edges without taking so much time the paint starts to dry. If you're planning on a single coat, you need to be very careful to inspect for "holidays," especially at the contour edges.
Use the 4" brush to remove excess paint. A "stippling dry brush" method is ideal because it picks up the paint and avoids leaving ordinary brush strokes marks. Just lightly "tap" the model repeatedly with the bristles until it seems that they aren't picking up any more paint. Then spin the brush dry (into a bucket) or wipe it off on a newspaper.
When this section of the model is cleared of excess, apply more paint so as to advance the "wet edge" of painted area. Unless you plan on putting on several coats and filling in most of the texture with paint, you should avoid letting the leading "wet edge" of the paint dry before applying more paint. [The overlap of new paint on top of dried paint would create a band with slightly reduced texture because it effectively receives an extra, unintended, coat of paint. If you experience this, however, you can reduce and/or eliminate it by applying one or more additional coats, thereby reducing the texture everywhere.]
One method to extend "working time," is to do your painting in a closed room equipped with a humidifier, which will slow the drying of the paint. Another option is to use alkyd paint.
If you want to preserve the foam texture as much as practical, one coat will probably be enough. If you want to obscure the texture, you may want to dry the model quickly with a fan and proceed with one or two more coats. After three coats, you'll probably notice that the change in appearance is less and less with each coat.
The 8 lb/cf foam does not require heavy painting for protection from normal handling. One reason for applying more than one coat, however, is to produce a surface on which you can paint details such as paths and then "paint them out" and paint on revised details. [A path on lightly painted foam is harder to obscure because each coat reduces the texture a significant amount.]
Before leaving the paint to dry, Take a close look for brush hairs or paint impurities. They can be removed more easily at this point than after the paint dries.
30" x 48" x 6"
1" = 40'
2' steps = .050"
roads depressed 1' = .025"
Roads and Paths:
The illustration above shows a study model on which the roads are delineated by .025" depression below adjacent areas. Although this is sufficient for some studies, it's practical for a patient person with a reasonably steady hand to paint the roads with a small brush and a single coat of full-bodied latex paint. Since paths are rarely depressed at this scale, it would be necessary to draw them lightly with a pencil before painting. A "study" alternative would be two apply "graphic tape" of the proper width to diagram paths.
Note on color:
The porous nature of the foam creates a pattern of dark "cell shadows." When the model is lightly painted, this tends to render the finished model a shade darker than one might expect from a paint chip, or an untextured sample. More coats fill in more cells and tend to reduce this darkening. In order to produce a "pure white" model, one would have to fill in all the pores - a task which is quite difficult on a stepped model. If color is critical, an experimental sample is highly recommended.
You can certainly use aerosol spray paints on polyurethane foam, including fast-drying lacquers. The most important thing is to protect yourself from the fumes. Beyond that, the masking can be challenging, but we'll try to pass along some tips on simplified ("nonprofessional") masking methods when we get a chance.
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