I covered my moulds in a light dusting of Macwax - a very common spray release agent that works well with Platgel pieces and many other materials. Despite this, I realised I was having problems getting the parts of the arm moulds off the cast because the Platgel had CHEMICALLY BONDED to the fiberglass. There is normally resistance when demoulding because of the suction created by the silicone - once the air creeps it's way back between the mould and cast, the mould parts eventually come away fairly easily. My arm moulds have no undercuts or heavy texture so there is no chance of this simply being a mechanical bond. The moulds were clean, dry (before the macwax was sprayed) and loosely covered with clingfilm to prevent dust n bits falling in - because of this I am confident that the gelcoat was normal and prepared properly for casting.
I realized the Platgel had bonded when I had been slowly knocking wedges all the way round the flanges, instead of new parts of the Platgel's surface becoming visible as the gap got bigger - it was the same area of silicone stretched between the two flanges. I attempted to pull the silicone away from the inside of the mould and was successful but it was incredibly stuck on there! Suction does not do this, it wouldn't do this even if I hadn't waxed the moulds.
I submitted my problem to a large community of industry professionals and hobbyists on Neill Gorton's Make-Up Effects 911 facebook group. This is exactly the kind of thing that they encourage people to post - so everyone can benefit from other people's experiences...
I had '0' explanations to my casting issues - go figure!
After eventually getting the first arm demoulded - I decided to slice the skin of the other arm as close to the existing seam line as possible in order to get the mould open enough to be able to grab the silicone and pull it away from the mould with force. I would then have to glue the silicone back onto the expanding foam core.
Here are a few hideous pictures of the damage caused:
I didn't take that many cos it is, quite frankly, embarrassing....
The damage on the first arm from the silicone being peeled off the mould looks like someone's dragged a disposable shaver over the surface (first pic). I will have to cover this in the seaming process as well...
Here are some snaps of me attempting to repair the right arm (the worst one). I used clear RTV bathroom sealant to glue the majority of the silicone back into the foam. I then used a fast setting silicone adhesive called ELASTOSIL A07, to glue the meeting edges together as neatly as possible. I will potentially have two seam lines to cover after I've finished...
