So my kid's high school is doing a production of Man of la Mancha. Much of this play takes place in a Spanish Inquisition (I bet you weren't expecting that) prison. Speaking of not expecting things, the theater director contacted me ten days (!) before opening night and said she needed some dungeon walls. I would make a comment about the flakiness of arty types, but I also happen to be in that category so I probably should keep quiet.
Fortunately, I already had a technique for creating what she wanted that I developed for doing my Halloween displays, shown here. I decided that while I was working on this that I would finally get around to documenting my process and sharing it with the Instructable community. You can use it for creating stone walls, stone blocks, or bricks.
The main material you will need is some extruded polystyrene foam. This kind of foam is a solid sheet, not like Styrofoam, which is made of lots of tiny foam balls compressed together. Styrofoam might work in a pinch, but it is much more fragile and probably won't give good results. The extruded polystyrene is sold as an insulating sheathing material and can be found at both Lowes (https://www.lowes.com/pd/Kingspan-Insulation-R5-Un...) as well as Home Depot (http://www.homedepot.com/p/Owens-Corning-FOAMULAR-...). It comes in a variety of lengths, widths, and thicknesses, so you should be able to find something that is approximately the right size for your project. Also note that if you need something thicker that what you can buy, you can easily glue several sheets together.
I was asked two cover 15 1/2 feet in width, and just under four feet in height, so I used two 4x8' 1/2" sheets for this project. Typically I will use the bluish-green (Dow?) foam from Lowes if I'm making a rock wall, and the hot pink (Owens-Corning) foam from Home Depot if I'm making a brick wall. Why? Inevitably, these will get a scratch or ding in them so I pick the color that will look the least bad if it get exposed. So I suppose if you were doing a hot pink fairy castle wall (I don't judge) the Owen-Corning foam would be the best choice.
Which brings us to glue. Note that may glues will not only not join foam to other pieces of foam or your backing material, but will actually dissolve the foam. This is a BAD thing. Additionally, nails screws and other mechanical fasteners will not work because they will pull right through the foam (although I will use nails as temporary fasteners for gluing). What I've found that works really well is Liquid Nails Heavy Duty Indoor/Outdoor Construction Adhesive (blue label).
Another thing you will need is some kind of backing to hold the foam rigid and allow you to use the aforementioned nails, screws and mechanical fasteners. In the mausoleum and gate I made in the past, I built a frame out of 1x3s. For this project, I used a sheet of 1/4" plywood. Either one will work and it mostly is the application that dictates what sort of backing I use.
The last thing you will need is paint. You will need a base coat as the primary color of the wall; grey for granite, brick red for brick, etc. This would be the color you would see if you were to squint or look at it from far away with your glasses removed. For this I use an oil based paint because it's much more durable than latex. With the base coat, a flat or matte paint is the best. You will also need a grey paint for the "mortar". If I'm doing a granite wall, I will use a lighter grey than the base paint. The last kind of paint you will need is either "stone" spray paint, or a set of acrylic paints (more on this later).
There are very few tools you will need for this project. I originally used a past-its-prime soldering iron for this, but that eventually gave up the ghost and I found a much better way to do this. First, you will need a propane torch. I like to use a high intensity torch head because I'm impatient instead of the typical "pencil-flame" torch heads. The high intensity ones, however, are more expensive and probably better for someone new at using one to go with the less intense version anyway. Going with the pencil flame will, however take much more time to heat and you will need to reheat more often. As far as fuel, I use camping propane (usually comes in a fat evergreen colored bottle). As far as I can tell, it's the same stuff as what they sell with the torches (propane is propane), but for some reason camping propane is cheaper ¯\_(ツ)_/¯. Also, I tried MAP, which they usually sell with the high intensity torch heads. I found that it was the torch head that made the appreciable difference and MAP should be just be called MEH.
Next you will need a steel rod. probably the best thing to use would be a long 3/4" diameter steel rod with a wooden handle, but since I have no idea where you'd get one of those (unless you made one), I just used a 3/8" carriage bolt. Why? I had it in my junk hardware bin. Best to use something you aren't too attached to since it will be covered with gunk (technical term).
You will also need a magic marker (color unimportant so long as you can see it against the foam) and paint brushes. For the paint brushes I used a 3" and 1" chip brush (this isn't fine furniture), and since I had a lot to cover and not much time to work (less than 10 days!), I also used a paint roller and the standard accouterments.
Edit: Optionally, a heat gun or hair dryer, a water spray bottle, and a rag can be used for more texture (more on that later).
Since you are going to be using hot burny stuff (technical term) you want to be using stuff that prevents you from getting burned... BADLY. Primarily this involves not getting burned by either the torch or indirectly through the steel rod. Like I said earlier, a long rod with a wooden handle probably would be best, but since I was using a simple carriage bolt, I made due with some welding gloves. At times it got a little unpleasantly toasty, but I never burned myself. You could also use some tongs or pliers, but I'd suggest that you have a way to lock them (e.g. vise-grip pliers) so that the red-hot steel rod doesn't go flying into something that doesn't like red hot steel rods being flung at them (e.g. your leg). It would also be a good idea to have a fire extinguisher nearby and ready to go, just in case.
The other bit of safety kit you will want is a decent respirator. Melting foam throws out some nasty fumes that I can't imagine would be good for your lungs or brain cells.
Okay, boring stuff done... let's get on with it.
Step 1: Plot Your Mortar Lines
How you do this will depend entirely on what effect you are going for. If you are doing bricks or blocks, you will want a regular pattern. If it's a rough stone wall, as I did here, you can take more creative license. Very simply what you do is use your magic marker to draw the negative area (e.g. the mortar) between your blocks, bricks, or stones. Here I just free wheeled it, just to get an idea of what I wanted to do. If I didn't like it, I could just draw a new line or even ignore it entirely when I got to the melting phase. It's just easier to lay this out with a magic marker and easier to change (the paint will cover the marks).
If you are doing regular objects, say bricks, it's much better to have a template you can use. When I do a brick wall, I cut out a brick shape in some scrap Masonite or plywood and trace the edges to draw the bricks in a regular pattern.
If you want to get really fancy (or don't trust your artistic ability), for under $20, you can get one of those paving or driveway concrete molds to use as a stencil. It's up to you how you want to do this. Have fun with it.
One additional aspect I would point out is that if you have a distinct front and back to your project, you should work with the blank side of the foam. I originally thought that it wouldn't matter because the paint would cover the print that the factory puts on one side of the foam. While this is true, the process that puts the print on this side slightly compresses the foam. I once made a fake tombstone before I realized this and if you look at it from a certain angle you can see the pink panther staring out of it. This somewhat takes away from the effect.
Step 2: Creating the Blocks, Bricks, or Stones
Okay, now the fun/dangerous part: Using the torch and melting some foam.
First off, make sure you have all your safety equipment either on or at the ready. Also work in a well ventilated area. Typically, I like to do this outside, but winter decided it wasn't done with us and I had to work on this mostly at night (did I mention I had to do this with only ten days(!) before opening night?).
All you have to do is heat up your steel rod with the torch, and when it gets hot enough, slowly drag the rod across your mortar lines. You may want to practice on a scrap piece of foam until you get a feel for how long you need to apply heat and what speed you drag the rod across the lines. Do not apply the flame directly to the foam. It will catch on fire... I've heard... it's not like I have any personal experience with this... umm... yeah. Anyway, did I mention that having a fire extinguisher nearby is a good idea? Yeah.
Since I was going for a rough stone wall, I decided I wanted to make the stones be rounded and be loosely fit together. For that reason, I etched out fairly wide mortar lines and knocked off the corners where the "stones" came together. For a brick wall, you would go with narrow mortar lines and only slightly curved corners and stone blocks would be somewhere in between.
Step 3: Sand the Edges
At this point, you should have some really gunky (technical term) mortar lines. Some of the edges will have glass-like melted foam sticking out. These should be relatively easy to literally knock off with a blunt edged instrument such as a file edge or butter knife. After that, get some 60 grit sandpaper to smooth and soften the edges. You don't need to go to crazy with this since imperfections will actually make the wall a little more realistic.
At this point you can get pretty creative with the overall appearance of the stones or bricks. For example, you could distress them with a wire brush. If I had had more time (did I mention I had less than ten days (!) to finish this?), I probably would have cut out some foam pieces to glue on top of the "stones" to give them more of a three dimensional and rough-hewn look.
Edit: I forgot to mention this step, mostly because I didn't have time for it when I did this particular build. When you've finished carving out the mortar lines, you can then use this technique to give the stones or bricks more texture. Using a spray bottle and a rag, spray water on the stones or bricks, then give them a quick wipe so that they are damp but not soaking. Do this to only a small section of the wall at a time so that it doesn't have time to dry off. Then, use a heat gun or hair dryer on the foam. Heat will cause the foam to melt, but the residual water will insulate the foam until it evaporates. This will cause the foam to melt and crack in a very organic, irregular way that will make it look much more natural.
Step 4: Attach the Backing
The backing can be attached either before or after etching the mortar lines. In this case I chose after because these were pretty big panels and it was easier to move them around when they were foam only. In this case, I trimmed a 4x8 sheet of plywood to the correct dimensions and then attached the foam to them with construction adhesive. I then placed both of the panels face to face, clamped them together and piled all the heavy things that weren't attached to something in my shop.
If I were doing a three dimensional box, rather than panels, I probably would build the 1x3 skeleton first, glue the foam to that, and only then do the mortar lines. Whether you use a 1x3 skeleton or plywood for your backing is also dependent on your application. Since I was just doing panels, a sheet backing made the most sense.
Step 5: Trim the Foam to the Backing
I left the foam at it's original 4x8 dimensions so that I didn't need to be very precise about lining up the foam with the plywood. When the construction adhesive cured I then used a utility knife to trim the foam to match the plywood exactly. Note that if you are making corners, you should leave the foam a little wider than your backing to make bevel or lap joints.
Additionally, when you are making corners or gluing foam to foam, you can press finish nails into the foam to connect the pieces together. These won't hold them for the long, but should hold long enough for the construction adhesive to cure bind the pieces together. Some more construction adhesive or spackle can be used to fill in the nail holes or any gaps as long as they aren't too large.
Step 6: Base Coat Painting
Regardless of whether you plan to use spray paint or if you plan to dry brush your finished work, you will first need to put a base coat of paint that represents the primary color of your stones or brick. In part, this is to conserve the amount of paint you will need to use for the finished work and reduce the overall amount of effort you need. For spray paint you will also need the base coat to protect the foam. Spray paint contains a solvent that will melt foam. This could ruin your work and create more fumes like when we melted the foam for the mortar lines. Of course you could work in a well ventilated area with a respirator for the fumes, and some people think the melting gives the foam a more organic appearance. There are two main drawbacks, beyond the fumes, that I see with this: First, you have no control over what happens. Second, the paint doesn't adhere nearly as well and you then have to apply even more paint. I'm just going to point out that regular paint is much cheaper than spray paint.
Normally I'd just paint the whole base coat with a chip brush, but since this was a pretty large area and I was pressed for time (did I mention I had less than ten days(!) to complete this?), I used a paint roller to cover the bulk of it and then used the chip brush just for the mortar lines.
Step 7: Decorative Painting
This is when you choose between spray paint and dry brushing. I honestly think that you can get much better results with dry brushing. Another advantage to dry brushing is that even when buying several different colors of acrylic paint (I usually get the the 2 oz bottles), it will still be cheaper than the spray paint. Additionally, while there are many varieties of textured spray paint, you are at the mercy of what you can find/buy (which as far as I know does not include red brick) while you can get pretty much any look you want with dry brushing.
I won't go into a lot of detail about dry brushing, but if anyone is interested, I could do another Instructable detailing the technique. Basically, you dip just the tip of your brush (typically an old cruddy one) in paint and then you wipe almost all the paint out of your brush. What this does is that only a small amount of paint gets speckled on the surface, particularly in the high areas. It's very simple and gives amazing results. For brick, I would probably use black, white, orange, brown, red, and some different shades in those color families. Stone would depend on the type of stone I was trying to create, but for granite, I would do black, white, different shades of grey, and throw in a little bit of brown and green.
Again since I had a large area to cover and not much time (did I mention I had less than ten days(!) to finish this?), I went with stone spray paint. There's quite a few brands and styles to choose from. I personally think that Krylon makes the best. Spray paint is also good if you aren't sure of your artistic ability since it's fairly fool proof. One other thing I would recommend is getting the spray paint at a hardware store over an arts and crafts store. In general, if you can get something at both a hardware and an art supply store, the art supply store will have jacked up the prices relative to the hardware store. Don't get me started.
Step 8: Painting the Mortar Lines
Almost done. Now all you have to do is go in with a small chip brush and paint in the all the mortar lines. I'll usually do this with a fairly light grey. Try to stay in the lines, but don't get to stressed if you get outside the lines. Mortar often gets in places it shouldn't as well, so it's just more authentic.
Step 9: Final Details
The picture here is just after I delivered the panels, but before they were installed. They were doing a lighting check though, so I thought it would be cool to get a picture of them on the stage.
There are some additional things that you can add to this process. For one, there are techniques you can use to harden the foam with resin. This can be a bit expensive, and I would only do this if this if it was something I was expecting to see a lot of wear and tear and it was fairly important. For example, If I were to do this for an actual art piece or if I were planning to create false exposed brick for a house.
When I want to age the look of the wall, I will get some brown and green point, thin it way down, dip my brush in it and fling it at the wall. If you do this, make sure first, that you do this outside or in an area you don't mind stray flung paint going and second, that you have your wall in the right orientation. Your really don't want drips to be going any other direction but down.
If this is going to be for a theatrical production, make sure you are familiar with local fire codes with regard to foam and wood. You will probably need to spray your wall with some fire retardant. If this is going to be an outdoor display, you should coat the wall with a matte spray sealant to help protect it from the elements.
One additional detail I will sometimes use is to get fake moss or vines to attach to the wall for an outdoor setting to add an extra level of to the walls. Get creative and have fun!