# carving mortar lines in polystyrene foam



## pmolsonmus (Jan 14, 2013)

Hi,

Have been searching the archives and found some good information, but not a specific answer to my question. Building a 32' x 8' wall out of 1.5" white polystyrene foam mounted on plywood for stability (I'm hoping to reuse this wall for upcoming shows).

Wondering if a grinder bit on a variable speed drill or a wire type bit will help to carve the mortar lines on a stone wall. I was thinking about something like this, but didn't know if the bit would rip apart the foam or the speed would melt the foam. Any help would be appreciated.

The wall I am replacing was done with a heat gun and torch and I have done enough research to know that is not how I want to proceed. (I've lost enough brain cells by being a teacher for the last 22 years!). Thanks in advance. Phil


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## Van (Jan 14, 2013)

I prefer something like this:
4" Shaft Mounted Polycarbide Abrasive Wheel

A radius wire wheel as opposed to a cup brush like you have pictured. You can even get versions that mount in a 3.5 - 4" grinder. The work well on foam as the handle on the grinder gives you added stability that you don't have with a rotary tool or drill motor.


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## JLNorthGA (Jan 14, 2013)

When I recently made a fireplace, I just cut the foam into blocks. I glued it to a base. I then used VSSSD (Van's Supersecret Scenic Dope) to act as mortar.


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## derekleffew (Jan 14, 2013)

I like a rotary rasp bit, like this: 

5/8 x 1-1/8" Pear Shaped Rotary Rasp, EazyPower 30086 - Abrasives & Polishers - Tool Accessories Tool Depot 24/7 in any drill motor. They come in different sizes allowing one to choose the width of grout line.
.


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## gafftapegreenia (Jan 14, 2013)

I've done it with a cheap laminate trim router from Harbor Freight with a 1" rabbet bit. Since we were doing several dozen sheets of yellow foam, we simply set up guide rails for the 4' runs and freehanded the perpendicular lines afterward. We then used a utility knife to give the blocks random shape and character.


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## Robert (Jan 15, 2013)

I tried this once and I cut the major lines with a dado blade on a table saw and then hand carved the other lines while smoothing the main lines. I found the walls hard to store and protect from damage that did not require re-painting. I guess I could have used a foam coating product to protect the walls, but that adds a lot of weight in my opinion. 
Another option I tried was a hardboard brick panel (Z-brick) and mess it up with some mud or spackle to break up the uniformity,but again a little heavy.


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## lwinters630 (Jan 15, 2013)

What type of stone look are you going for?


use 1/4 inch luan or tape on plywood cover with drywall compound mixed with paint, lift tape in 5 min


vinyl template and VSSD , or tape on plywood cover with drywall compound mixed with paint, lift tape in 5 min


carve with chansaw wheel on angle grinder ( or any other tool ) and cover with VSSD


chemical etch with brush on 3/4 pink foam out doors


Bed sheets soaked in paint over wood and wire frame

Add flame retardant when needed.


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## pmolsonmus (Jan 15, 2013)

Thanks for all the replies.


I am using 1.5" foam to give it depth. We're in a black box and the plan is to use the wall for a musical revue of songs from shows set in castles and then reuse in the future for a madrigal dinner every other year.


We will have windows cut in the foam for stained glass windows that were made out of colored tissue paper and heavy duty construction paper. They are beautiful and have lasted over 15 years with care.


Here is a pic of the madrigal court with the wall and stained glass in the background


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## lwinters630 (Jan 16, 2013)

Hey pmolsonmus,

You are only a few hours form me. We are doing Les Mis HS version this spring right after Easter. It will be based on the newer 2010/12 version stage wise. If you are interested in purchasing our set when we are done, or just want to see it, give me a PM.

Have fun with your show.


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## pmolsonmus (Jan 16, 2013)

Thanks for the offer.

We already have materials and have begun construction. Show opens 2/21 and the plan is to be able to build for the specific space and reuse for a Madrigal dinner that we do every other year.

Phil


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## BrianWolfe (Jan 18, 2013)

I agree with cut the foam into blocks and attach to a substrate that has been painted with sand paint. If it is necessary to cut the lines I'd use a router with a straight or v grooved bit.


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## pmolsonmus (Jan 20, 2013)

Hi, thanks for all the input. After working with the foam for awhile, I found it easiest to draw in lines and use a poly carbide wheel on a grinder to carve out the lines. (Thanks for that suggestion). I then went back over with the rasp bit for shaping and gouging interest, in addition to kneeling, pressing and stepping on the foam.

I am ready to paint and I am planning to add pre-mixed joint compound to help seal the foam and minimize the amount of coats it will be necessary to cover completely. I did the search on VSSSD but I don't think I need the texture for this project. Am I wrong thinking this? 
If I am just adding joint compound how much is necessary per gallon of paint to seal without lots of cracks or flaking off?

My plan is to seal and cover with gray and come back with black for grout lines and black, brown and darker gray with sponge and rag techniques for the stones.


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## pmolsonmus (Feb 18, 2013)

The wall is finished. Using LEDs as uplighting for the shears and some small LED striplights for backlighting through the windows will upload a pic when lit. But even in basic light, the wall turned out great. Thanks to everyone for the help.


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## techieman33 (Feb 18, 2013)

Great job on the wall.


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