In the November/December 2012 issue of Concrete Decor, I presented a simple process for casting a sink using fabric forming.
The process for “free-forming” a fabric mold — for all kinds of creative, ripply results — is a little different but will include many of the same steps as basic fabric forming. The biggest difference is that you will be working on the outside of the mold first.
We used fabric-forming to create a free-floating shelf.
1. The armature
This was the easy part. As we were making a shelf, our armature only needed to be a flat platform raised high enough for us to drape the fabric over and achieve our desired height. We simply used a piece of melamine cut to the size desired for the shelf — 12 inches deep by 36 inches across — with 24-inch legs.
We then sealed the raw edges of the melamine with tape. Given that we will be adding a Bondo-resin mix to this surface and sanding, there was no need to worry about the lines from the tape transferring to the mold.
After applying the tape, we applied two coats of mold release wax, buffing both coats. This will ensure that we can remove the armature once the fabric has stiffened.
Note: You can ease or round your edges of the armature board if you do not want a crisp corner on the final top.
2. Forming the fabric
Now that the armature is ready, we prep to wet out the material. There are a couple of ways this can be done. You will need to have a few things ready when you do this method (beyond those in the Basic Fabric-Forming Tool List). Here is the process and the things you will need.
First, we will lay our fabric on the armature and cut it to the desired size and shape.
Allow it to drape about 3 inches longer than you intend the piece to be, in case you decide to cut the bottom of the cast piece to clean up any raw edges. This is not always necessary — the need to do it is determined by your mix and your ability to cast vertical surfaces. If you can cast vertically to a finished edge, make your piece the exact size you want. Then trowel to a finished state at the top of the mold.
Once your material is cut and ready, get all your tools ready for wetting the material, the next step. You will need a hot glue gun warmed up and ready to glue. You will also need extra armature pieces to help hold some of the drapery in place. You can use PVC pipe of different diameters or wood dowels — or anything, really — to hold your fabric in place. I prefer the PVC as it releases from the mold much more easily.
3. Wetting out the fabric
Once all your pieces are ready, you will then wet out the fabric. There are two ways to do this. One is to use resin and the other is to use an acrylic polymer. If you use resin, you will only have a small amount of working time with the material before it stiffens up, so this is best reserved for smaller molds. Make sure you are ready with your design and your extra armatures, or you may be wasting material and starting over.
Wetting out the fabric using resin
To create our mold we mixed a quart of resin with 1 percent of a quart (about 10 cc) of MEKP hardener. This is critical to have enough working time with the material. Once the resin is mixed, we saturated the material in the resin, then draped it on the armature.
Once it was in place, we began to tug on the fabric in different areas to get our drape to the style we wanted. The weight of the resin (or polymer) is the trick here. The weight will pull the fabric down, giving you the draped effect.
You will now need to use the glue gun to tack your fabric in place to hold the shape. You can do this with the hot glue and your extra armatures. For our mold, we used the hot glue and tacked the fabric onto the armature that was already there.
For the flares on the fabric, the areas away from the armature, we glued PVC pipe to the base of the armature to hold the fabric in the place we wanted it.
Note: This is where the different sizes of PVC will come in handy. If you want a bigger curve, you will use a wider pipe. Also, remember to wax the pipe so that it releases from the mold.
Once the armatures were in place and the fabric in the basic shape we wanted, we did the final details.
There are areas in your mold that will have wrinkles that you will not be able to cast into, either because they are too tight or too undercut. There are a couple ways to deal with this.
On the top of the mold, the part that will be the shelf, we cut the fabric and overlapped it on top of itself. We used a little extra resin here to make sure they bonded. We also used a little fiberglass matting here to ensure that the mold holds together.
For other areas that we could not manipulate without changing the design, we used Bondo in the mold to shape these wrinkles out. We also used Bondo where we cut the fabric and laid it over itself. Just a little bit of body work, so to speak.
Once the fabric had firmed up and we had locked in the shape, we applied two more coats of resin to make the fabric rigid. This was applied to the outside of the mold and the fiberglass matting in the cut areas.
Note: We also used the fiberglass matting in areas where the mold needed a little more firming up, such as the corners and the area we had cut on the top. Be careful about using matting in areas that may have undercuts. If the matting is pinched between two folds, you may have to break the mold to get it out of the cast piece.
We applied one coat of the Bondo-resin mix on the exposed area of the inside of the mold. This gave us a little more rigidity to ensure that the mold was solid enough to remove from the armature.
We also put a layer of matting on the flat part of the shelf along with a couple of 1-by-2 wood strips to hold the mold flat.
Once we removed the armature, we went through the Bondo-resin finishing steps that were laid out in the November/December issue (and can also be found online at ConcreteDecor.net), applying about four coats total and sanding to a 320-grit finish. After wax, we were ready to pour the concrete.
Wetting out the fabric using acrylic polymer
Using acrylic polymer is easier but it takes a little more time, as the polymer takes longer to dry and stiffen up. Following the same process as you use with the resin, you will wet out and drape the fabric on the armature. It will take about one to three hours for the polymer to dry to a stiff form. Once it is stiff, you will need to apply several coats of resin to the fabric to get it to a more rigid state.
With this method, using the polymer, you will have time to work with the fabric and get it to the shape you want. However, be aware that the polymer will only stiffen the fabric to a firm state. Adding resin to the fabric will be necessary to make it rigid, and this can be a trick.
The fabric can wrinkle on you if you brush too hard while applying the resin, so be gentle. After the first coat of resin, your material should be firm enough to work with. Once it is firm (after two or three coats of resin) you can follow the usual resin-wetting procedures to finish.
Note: You can use just about any polymer to make the fabric stiff, but do a test on a small amount of fabric to make sure that the one you use will give you the firmness you want and will bond with the resin. A high-solids polymer is the way to go. There are also specialized products designed to stiffen material, and you can find these at your local craft store.
Tommy T. Cook is the founder of Gnomeadic Arts and co-founder of Seattle-area company Absolute ConcreteWorks.
He is a trainer, consultant and artist in the precast industry. Reach him at firstname.lastname@example.org.