A couple folks had asked me what it is that I do about my foamcore edges in various dioramas. Years back, I would leave them exposed, which would be so obvious especially when taking pictures! Part of the illusion of dioramas is to remove the signs of what materials are being used. So I thought I'd take some pics of a couple methods I use, which are very easy but a little tedious. It's worth the effort I think, to hide the fact that it's foamcore being utilized.
Hiding Foam edges
The most common place to do this would be for the floor of a diorama, or a wall that ends in the direction of the viewer. A strip of posterboard hides the foam interior nicely, and takes spraypaint as well as the foam board itself. Here's what I do:
Take the foamcore edge that is going to face the viewer, and gently run a hard straight-edge along it to depress the foam beneath the upper and lower posterboard layers a little. The L-shaped metal straight-edge I use is great for cutting nice lines in anything.
It should look like this when done, just a posterboard thickness lower than the upper and lower coating.
Next, take a sheet of posterboard and line up your straight-edge so that you'll cut a strip that is the width of foamcore, like the piece I'm holding to measure it.
Here I have the strip of posterboard held next to the foamcore groove where it will soon be placed. (Note: my posterboard strip was not cut long enough in this pic as you can see, but normally you would have it the same length as the board).
Hot glue gun time! Apply a thin sliver of hot glue quickly into your groove, about an inch to 2 inches at a time.
Here it is about halfway attached. You want to start laying the strip into the groove pretty quickly before the hot glue dries and leaves bumps. Have the strip ready to go right next to you so you can quickly set down the hot glue gun and pick up the posterboard strip.
Once completely attached, you can spraypaint it along with the foamcore surface and get one even color (without worrying about the foam melting from the paint).