Part 7: Making a fibreglass mold

So we come to the most stressful, difficult and dangerous step so far: making a fibreglass mold. When I say dangerous, I don't just mean that if it goes wrong you can end up erasing all your hard work so far, but in a more literal sense. The polyester resin and glass fibre matting, not to mention the various sealants required, are noxious chemicals and require some mid level personal protective equipment, and the powertools needed in the later stages are obviously capable of causing some pretty nasty injuries. I personally used a ton of disposable latex gloves, a respirator with replaceable filters, goggles whenever dust or shards were involved, a paper suit to protect my clothes as well as changing clothes to avoid tracking anything indoors, and I worked in a large garage, often with the door open and a large fan for venting.  Even so, I am not a health and safety professional. Please if you do take any inspiration from this blog and try anything for yourself, do your own research and look for advice from real professionals. Also, I am missing out a few steps, this is not an instruction manual on using fibreglass (though I hope it is still of use to people) 


So with that slightly paranoid diatribe out of the way, onto the actual processes and pictures

I begin by sealing the sculpt with clear lacquer, I used a brand intended for wargaming miniatures. This seals the still somewhat wet clay, gives it a non porus surface for the clay wall to stick to and the resin to sit on, and makes it a little more durable and less liable to taking surface impressions from the pummelling it's about to endure

Next I have to decide where the dividing lines between the parts of the mold will end up. Fibreglass is pretty rigid, so I can't afford for any individual part of the mold to have any substantial undercuts. In the end I went for four pieces, on advice from a friend and my own ideas, two for the face and two for the back of the head. Each part of the mold needs a substantial lip, so we make clay walls from cheap water based clay along the dividing lines

making the wall resting on a scaffold of scrap wood and plastic. I didn't do this too well to be honest, but it seemed to just about work

the clay wall has small clay "keys" so the parts of the finished mold match up when it's bolted back together

on areas where a scaffold can't be built, the wall is braced against the sculpt using more clay. The sculpture is sealed with the spray lacquer mentioned earlier, so these buttresses should just life off when I remove the wall

Next, I seal the sculpt again with PVA sealant (poly vinyl alcohol, related to kids white glue but NOT the same stuff, you get this from fibreglass suppliers), applied with a spray gun. The spray gun and compressor both belong to my roommate who was also instrumental in telling me exactly how to do this, and helping out on those few steps where two pairs of hands are necessary.

After a final spray down with a wax mold release, it's time for the first coat of resin. The base layer on this mold is a "bondo coat", made from a mixture of car body filler (I actually used a non branded version) and Polyester resin, with both catalysts added after mixing. These are mixed in a ratio to obtain an emulsion paint like consistency, (for me it was on the order of 50:50 though it may vary by brand), then poured onto the sculpt and worked in with a soft brush

this is actually the bondo layer on the *second* stage of the mold, but I was so stressed and unsure when doing the first half I forgot to get any pictures

After this, once the bondo layer has mosty but not completely cured, the major indentations around the nose, eyes and sharp edges of the clay wall are filled with resin thickened with cabosil (fumed silica), a peculiar fluffy powder that is extraordinarily lightweight and when mixed with the resin gives it a toothpaste like consistency. This avoids any big gaps in the fibreglass layer, or the risk of having large volumes of polyester resin without any strengthening fibreglass, as the resin is very brittle when used alone and the cabosil adds a lot of strength

The next step is the fibreglass. Because polyester resin is sticky and dries quite fast, I wasn't able to get any progress photos during this stage, but there are plenty of filmed examples online. In any case, I applied three layers of chopped strand matting, with polyester laminating resin which is worked in with a paintbrush like a kind of stinky irritating papier mache. In case anyone is interested, I catalysed 2-3% at 10 degrees C, which seemed to work well. Oh and by the way, the catalyst is also an explosive, if this didn't sound dangerous enough already.

chopped strand matting, this comes on a roll like fabric and can be torn into small pieces for laminating. The strands are only loosely attached to one another and slip easily, making this form of fibreglass easy to tear by hand and suitable for covering quite convoluted shapes

there are still a lot of fibres sticking out of the edges of the mold piece here, that's fine, we'll be trimming those off later

After completing one part of the mold, The process is repeated on the other sections. The clay walls need to be removed and the resin underneath cleaned, then everything is sealed with more PVA and sprayed with more wax. I also add a few small clay lozenges to act as "pry points" where a screwdriver can be inserted once the mold is completed and trimmed

Each time I complete a "lip" on the mold with fibreglass on both sides, I drill a few holes and bolt it together, to prevent shifting while working on the rest of the piece. In this case I used M6 (approx 1/4") bolts and wingnuts which can easily be tightened and loosened without tools. Eventually all four edges are bolted up like this.

Finally, to make the back of the mold, the entire piece has to be tipped forwards. I was lucky that my work space was made from two old wardrobe doors (found in a skip), so the front lip was able to rest in the gap between them, with the front of the mold resting on two large foam pads and the back of the lifecast elevated on pieces of scrap wood. This was one of those two person steps mentioned earlier, between the plaster base, the clay and all the mold making materials this thing is HEAVY. In general, when doing large heavy molds it's always handy to have a bunch of scrap around, literally everything I used to support this mold was salvaged material from skips or neighbours throwing it away. Fibreglass and resin will pretty much wreck whatever you are working on, so it's wise to try to reduce costs and waste by using things that were destined for the bin anyway and giving them one final lease of life.

And that's it. All the wet stages of making a fibreglass mold. Next we finish bolting the thing together, trim off the excess then pry the mold off the sculpt, followed by a good wash, a week or so sitting out to finish outgassing, and a lot of fiddling and cleanup, but this entry is already long enough so I'll leave it here.