Possible solution to getting realistic concrete - Model Railroader Magazine

I've got a technique I call speckle painting that works well.  I've been meaning to put up a webpage tutorial but haven't gotten around to it.  If you use plaster you won't need a base coat, but other materials may need a base of white or off-white (sort of a 'bone' or 'eggshell' color.  Don't paint it gray.) 

You're going to need a couple of shades of brown spray paint, one fairly light and one medium.  Avoid the dark chocolate shades. I used Valspar Sagebrush for the lighter shade and Krylon Brown Boots for the darker shade.  There are other similar shades out there: khaki; putty; leather saddle; etc.

Then cover the floor of your workshop with newspaper and lay the concrete parts on it. Start with the lighter shade of spray paint and hold it about 3 or 4 feet above the floor and give a few quick bursts.  The paint particles will settle onto the parts giving it a speckled look.  The change can be subtle so inspect the parts every few bursts.  You can always add more, but can't take any away.  Be sure to keep the nozzle clean or it might splatter on your work.

Then give a few bursts of the darker brown.  You probably want less of this one than the lighter shade.    Freshly poured concrete shouldn't have much paint.  The older the concrete is supposed to be, the more paint you'll need. 

After you have the effect you want, then give it a wash of india ink and isopropyl alcohol.  Plaster really soaks this stuff up so you'll need a thin solution (start with about 1/4 teaspoon ink per 1 pint of alcohol and go from there.)   If you're using plastic or foam you can go with a heavier dilution.   For small parts I find it's easier just to pour the wash into a shallow container (like a margarine tub) and dunk the parts.  If your parts are too big to dunk, use a wide brush.  Don't use a spray bottle on plaster; it will leave spots.

Here's a few pics.  This is plaster with varying degrees of paint and wash.

Here's the mold I used to make the castings.  It's two pieces of 2mm Depron foam, a base piece and an upper one with a rectangle cut out of it.   You can see that the plaster casting really captures the texture of the Depron.  This mold has been used at least half a dozen times so it's getting a bit rough.

I like the plaster a bit watery.  The Dap plaster I used called for 2 parts plaster to 1 part water.  I used 1.5 parts plaster to 1 part water.  After pouring the plaster in I layed a small pane of glass over it.  This forces the plaster into the crevices and ensures a flat bottom.

Let it dry for at least an hour, preferably longer.  To remove the casting I held the mold in my left hand with my fingers on the underside, directly below the casting.  Then I carefully use my right hand to pull the outer edges of the mold down and away from the casting.   You'll need to work your way around the entire thing a few times.  Eventually the casting will work loose. 

The plaster I used is a light tan when freshly dry, but after a few days fades to a bone color.  You'll want to let your plaster cure completely before painting it, or else the color you thought you had may change on you.

If you don't want to be bothered with plaster you can paint the Depron foam with convincing results.  Paint it white with water-based craft paint (to protect it from the spray paint).  Then speckle paint and give it the ink wash.  The downside to using foam is that it's not very durable.  It will dent easily if not handled carefully.

You can also use speckle painting on brick buildings.  Paint a base layer and then find some spray paint that's a little lighter or darker (or do both.)  It will give some depth to the brickwork.

Steve S