How to: make a fiberglass mold! (also: clay walls and a quick tutorial for silicone molds!)

This tutorial is to go along with my Queen of Blades tutorial series, but can be used for any fiberglass molds! It also covers silicone molds really quickly, since you can use the same clay wall to make a silicone mold. I’ll cover stone molds next!

Ok. Time to learn about one of my least favorite things to do- making a clay wall to split the mold in half. This is tedious work, but necessary. Take your time with this!

What you need to do, is firstly have a clay cutting board. We made one pretty easily by glueing two pieces of precut square wooden dowels to both sides of the laminate board with Loctite Power Grab. Make sure to clamp the sides down and let it cure before you do the other side. We put 1/2 inch dowels on one side, and 3/4 inch dowels on the other, and both sizes have served us well.

You will also need a clay cutting wire, which is just a long wire with handles on both ends so you can pull it across the clay using the board as a guide to cut slabs of clay, and Kryolan Crystal Clear spray, to seal the clay with, as well as Kryolan dulling spray if you’re making a stone mold. You can find dulling spray at most art stores.

Okay. Now:

(I will be using pictures of other pieces I have made clay walls of since I don’t have as many as I’d like of the stone hands being molded)

1. If you have a large sculpture like a mask, or body suit, you can make your clay wall with the sculpture standing up. Otherwise, we liked to have our sculptures laying down. If you are laying down your sculpture, and if it has clay on it (we’re doing the stone hands right now, but more often in the future you’ll have clay on it when making the wall) make sure to put down a thin slab of clay with plastic wrap over it to protect your sculpture. Don’t bother with the arms, they’re plain stone, so just lay them down on some cardboard (to protect your work space. Newspaper works too!). It may be necessary to put pieces of wood or foam around your sculpture if you’re laying it down to help bring the clay wall to the middle point that it needs to be.

2. Before cutting the clay, take a marker and put dots in a line that will prevent undercuts in the mold that will possibly damage the mold. (Undercuts are areas where the mold wouldn’t be able to be pulled apart from the sculpture; I like to imagine the mold being pulled straight out from the sculpture, and if there are any spots where it looks like it would catch on the mold, then those are undercuts.)

2. Using a clay cutting tool, cut slabs of clay about 1/2 inch thick; lay them flat and cut strips about 2 inches wide and as long as you feel comfortable with for each piece- for larger molds, you can use pieces 4 inches long, but for smaller molds with lots of curves, you can use shorter pieces.

3. IF you are making the clay wall on a sculpture, seal the sculpture with Kryolan Crystal Clear to help keep the fiberglass or silicone from sticking to it. If you’re sealing water (wed) clay, use 7-8 coats of sealer because fiberglass doesn’t do well with moisture at all, and the Crystal Clear helps prevent that. If you’re using oil based clay, then only use 2-3 coats.

4. Place the strips of clay along the dotted line you made along the sculpture, pressing down hard enough to get a good connection between the clay and the sculpture (or stone) but not hard enough to damage the sculpture with the clay. Make sure the clay wall is perpendicular to the sculpture, and the edge to be as clean and smooth as you can make it. Try to make the wall as close to 90 degrees as you can.

From Special Makeup Effects: “A perfect wall will have no gaps where the water clay meet the oil clay.”

5. Continue adding the clay wall all the way around the sculpture. In some spots, you may have to get creative. If you have some hard curves, cut triangles out of the clay to help you curve the wall around. You may have to add in triangles in spots like in between the fingers. If you have spots that aren’t supported underneath and are weak, then shove some bits of wed clay under there to help keep it from falling or sinking.

6. Now for the hardest part, to me. Use a small dental tool, or a small flat clay tool, and smooth the clay seam until it’s perfectly closed and neat. The wall should be as smooth as possible, and flat all the way around the sculpture.

7. Your next step is to clean up the clay wall, cut around the edge of the clay to make a nice, clean edge all the way around the wall. Then, use a small object with a rounded edge (I used the top of a lipstick case), or a round wire clay tool to create keys on the all around the sculpture. This is the help the two halves of the mold fit together perfectly. Go ahead and put a wall of clay around the end of the arms or sculpture too, as you see in the above picture, to prevent the fiberglass from running down where it doesn’t need to go.

7a. IF USING SILICONE: If you’ve decided to go with silicone instead of fiberglass, use these next steps. You want to make a wall vertical to the clay wall, you can use clay as long as you make sure that you have a good seal. If you don’t have a good seal, you will lose a lot of silicone. Another material you could use on the wall could be cardboard, though if you use clay, it would be easier to maneuver around the wall you just built. An example of the walls I’m talking about is in the photos below, make sure that the wall is high enough that pouring silicone in will cover the stone arm completely, with at least half an inch left above it so that the sculpture isn’t too thin.

Mix your silicone according to the instructions on the bottle, and pour it into the mold. An easy way to make sure there are no air bubbles is to pour from a distance, like in the photo below.

Let the silicone cure for the amount of time it says in the instructions, then unmold it. Make another wall around the other side, don’t forget to seal the clay again with Crystal clear, and seal the silicone with silicone release so that it doesn’t all stick together- don’t forget the release! Pour the other side, and now you have a hand mold!

Now for the fiberglass tutorial:

1. Seal the clay with 7-8 coats of Crystal Clear- thin coats, so that the moisture from the clay gets nowhere near the fiberglass, because it could prevent it from curing.

2. If you’re doing a mold that doesn’t require you to pick up any detail, just the shape of the object (such as the mold we did of the stone cast of my hands), you don’t necessarily need to use gel coat, just resin and fiberglass cloth.

It’s easier to have two people working at the same time on this, one to wet the cloth with resin and one to apply the cloth to the hands. Mario did a simple tutorial on the resin here, so check that out if you’re unsure of how to work with resin for fiberglass, and how to figure out how much catalyst you have.

3. Always release your mold. After you seal it with Crystal Clear, use a good fiberglass mold release on top of it- we usedEpoxy Parfilm spray. It’s good to lightly brush some talc over top of the mold release as well, to get rid of any extra moisture.

Also, always wear gloves and a  respirator when working with fiberglass. We prefer the ones with cartridges like this, if you have on a respirator and you can smell the resin, then it doesn’t fit you well enough, or it is not the correct respirator.

If your sculpture is of a glove or mask or anything with much detail, you want to use gel coat first. We used a clear gel coat, you may be able to find it at a marine supply store. Follow the instructions with the packaging- it uses the same catalyst as the resin we use for the fiberglass matting- and gently brush a thin coat all over the clay wall and sculpture. You only need one good coat, but sometimes with the gel coat it can slide around if it doesn’t want to stick to your sculpture, so keep an eye on it, and take it slow. I think ours took up to an hour to cure enough to put the fiberglass on top.

IMPORTANT: Once you start with the gel coat, you absolutely need to finish the entire mold the same day. If the gel coat sits for even a few minutes too long, it WILL crack, and ruin the mold. You will have to start over, or very carefully peel it off, repair the sculpture and start over with the mold. Only let the gel coat sit until it’s just barely cured, then IMMEDIATELY start with the fiberglass cloth on it. I promise I am speaking from experience, and having this happen sucks and sets you back several hours.

4. If you are working alone, work quickly, and know how much resin and fiberglass cloth you can use before the 20 minutes it takes for the resin to start to gel. With two people working, our usual amount of resin was 12 oz for me (I was wetting the fiberglass matting with the resin) and 4-6 oz for Mario, who was wetting the sculpture with his resin and using it to help adhere the matting to the mold.  Be organized, have your matting precut into sizes you can work with, I think our usual size was 3 by 5 inches? Around that size. We used laminating resin from Tap Plastics, and silicone mats from baking stores to lay the wet matting on, so that when it cures, you can just peel the excess resin off.

Measure out your resin, then quickly measure your catalyst and pour it into the resin, stirring quickly for at least 45 seconds to make sure that the catalyst is evenly distributed. We LOVE this dispenser for catalyst that we got from Tap, it has a handy measuring device in the top and is much better than measuring out drops.

5. Lay out your pre-cut strips of fiberglass matting, and saturate them with the resin using a chip brush. (Definitely buy boxes of 1″ chip brushes, so useful, we went through at least 6 boxes) Only let them sit for maybe 30 seconds or so, if you let them sit for too long without using them, they will start to break down because the resin is what breaks down the fibers in the fiberglass and makes it flexible. Spread a thin layer of resin on the sculpture, then lay down the wet fiberglass on it, using a chip brush to make the matting flat to the surface and to get all air bubbles out. Air bubbles are bad! Repeat and do at least 3 layers of fiberglass matting all over that side of the sculpture and clay wall. You may go through more than one batch of resin.

For the body suit, we did three full coats of fiberglass matting on the body, and four on the wall, because it was such a big mold and we wanted to make sure that it was strong enough. A note on the body mold: it took us two 17 hour days, with only breaks for some sleep and quick food to finish it, I honestly don’t even know that this mold could be done in one day unless there are a team of at least 5 people working on it together. It was exhausting.

6. Let that side of fiberglass fully cure before you do the other side. Sometimes, pulling away the clay wall is a pure pain in the butt, for the body suit it took us three hours to get all of the clay wall off of the other side of the mold… it was our own fault, really, for not releasing it enough I guess, either that or it was too cold and the clay got harder therefore making it harder to remove. Either way, it sucked.

So, when that side cures, carefully turn the mold over- be careful not to cut yourself on the hardened fiberglass- and start to carefully remove only the clay wall, be very careful not to damage the sculpture in any way. If there are any spots that are being especially difficult and they’re really close to the sculpture, and only if you used chavant clay on the sculpture, use some water and a small, soft brush to help get the clay loose and out of there.

7. If you’re doing a glove or arm or something that has an end like the end of the arm, put another clay wall right on the end like you did on the other side to help prevent the resin from dripping down and making it difficult to take off. Cover the exposed clay with Crystal Clear, then the wax release and talc, and do the other side just like you did the first side with the resin and matting.

Opening Fiberglass Molds: 

Your new best friend in opening fiberglass molds are your drill, popsicle sticks, screw driver and a vibratory cutting tool. To prepare your molds for opening, first drill holes around the mold that will be used for holding the mold shut during storage and casting, and will help keep it shut and also from warping. We used a 1/4 inch drill bit for 1/4 inch bolts and wing nuts. Make sure the bolts are long enough to go through all those layers of fiberglass, I think ours were at least an inch long. Better to be safe! Make sure the holes are about every 2-4 inches on a smaller mold, we put on maybe every 5-6 inches on the body mold, and of course make sure they’re not right where the keys are, or you’ll ruin the keys.

Next, we liked to go ahead and screw all those bolts in nice and tight, so we could clean up the edges with a vibratory cutter. You don’t have to always screw them in, apparently we didn’t for the glove molds. Then clean up the edges!

Find a spot where you can see a definite line where the mold would part, and carefully stick a screwdriver in there. Once it’s wide enough open, stick a popsicle stick in there, and continue to move around the mold. For the smaller molds it can be quite easy and quick, for our body mold we had to account for the large area between the legs, and ended up hammering a long yard stick in between to help seperate the mold. It took us maybe 4 hours to open the body mold, and this is also a task better suited to more than one person.

You will hear some popping and cracking, this is normal with fiberglass molds, it’s just the mold halves separating. Finally, you’ll get the mold open and you’ll have the fun task of cleaning out all the clay…

If you’re cleaning out chavant oil clay, D-Limonene  works wonders, just scrape out as much as you can with a small tool, being careful not to damage the mold, then go in with some of the D-Limonene and a chip brush, and clean it out. Please make sure to wear gloves when working with D-Limonene, though! It can eat right though a plastic cup, too, so put it in a glass container if you’re using a small amount at a time. If you’re cleaning out water clay, just scrape it out and use a hose with some pressure to help get the rest of the clay out.

Oh, my goodness. What a long process just to get some molds that we can finally use to make something we can wear! How exciting!

Make sure that your molds are completely dry before you put them back together, and also always store your molds bolted shut, or they will warp and not fit together correctly.

I think that’s it, as always, please let me know if you have any questions or concerns!

12 thoughts on “How to: make a fiberglass mold! (also: clay walls and a quick tutorial for silicone molds!)

  1. Pingback: Queen of Blades tutorial Step Six: Molding the body suit | Arms, Armor and Awesome

  2. Pingback: Queen of Blades tutorial Step Seven: Molding the smaller pieces (or: how to make stone molds!) | Arms, Armor and Awesome

  3. do i use “tooling gelcoat” or “polyester gelcoat”, and lastly, do I need to put PVA on my clay sculpt or just use the crystal clear and epoxy release you suggested? Please help…

  4. Thank you for replying back to me so fast. Sorry, I have been out of the country. I just want to make sure I have the right items, because I am wanting to make a mold of the head sculpt I am working on for a lightweight and multiple castings mold. I love you guys blogs, awesome, detailed and easy to understand. please let me know if you have a facebook that I can add you.

  5. Just so I have this right, after I get my diving wall all done for my sculpt I1st coat the one side with crystal clear, then Mold Release on everything including the diving wall, then I do the Gel Coat (Should Let it cure for maybe 30 mins before applying the resin? I’m going to be molding a mask.) and then I apply the resin and top with fiber glass and more resin?

    Does it matter what kind of resin I use? Thanks a lot for this tutorial, it’s like the only one out there.

  6. Hello! I am so glad i found your page on fiberglass molding! I was beggining to give up looking for turorials. Thanks so much for all the info. I am trying to create light weight molds of my dragons so i can bake foam latex in them. I was wondering what brand of materials you used? And where i can get them? Thanks:)

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