So, I’m now making the first side panel, which is an intersection of five planes. This is a bit of a step up from my other panels which have been flat.
I have another post to go into the making of the mould but this one is more about the chemistry. As I’ve said to Warren when being frustrated by this before, getting this right is a mixture of talents, namely being a baker, chemist, structural engineer and tailor. He’s got all those skills in spades, and I have few.
So, The mould shape was made from slabs of fibre-board (more on this in another post) clamped to the car and then bonded on the back with glass and poly resin. Then I decided to face the front of it with bog (car body filler) in order to get a reasonable finish.
Now for the trixy bit: epoxy and styrene (the stuff that smells sweet in bog) absolutely don’t mix at all, because the styrene gas breaks down the surface tension in the epoxy and it runs away. Remember this bit for later.
The first approach to preparing the surface was to spray tack some release film to it and mould straight off that – it wouldn’t be super shiny and cosmetic but would be very quick to prepare the mould. Spray tack was applied and then release film. Initially it looked lovely and shiny. However, after a few minutes bubbles were appearing under the film, due to styrene gas coming off the part; it was very green at this point. Nadgers.
So, I call Warren and get advice. I don’t have ready access to equipment to spray high-build primer or 2k clear-coat, so the approach I can use is to mix glass microspheres in with the resin to bulk it out and make it easy to sand, then paint it on.
It went off and separated. Three reasons – firstly I didn’t clean the spray-tack off (I assumed epoxy sticks to anything therefore spray-tack can eat my shorts), and secondly didn’t appreciate how sensitive some of this chemistry is. Thirdly it looks like the styrene gas is still bleeding through.
So, how to fix? Go to Warren, sand all the crap off and spray with 2k clear coat.
Job done, and I now have a reasonable mould.
- It takes ages for the poly resin to stop gassing (weeks). Poly is the resin of choice for mould reinforcement though because it’s 20% the price of epoxy
- it’s a lot of work with an orbital to get that lot off
- make friends with a local body-shop who can do the spraying as a side-job for beer tokens.
- It wasn’t necessary to apply the bog to the surface all over, and as thick as I had. All this meant was that I had a mildly uneven, slightly wavy surface. It doesn’t affect the integrity of the part, which fits nicely but it does show up if the sun is bouncing off it and you have a critical eye.
I have now pulled a part from this and it fits well. what’s more, it is 546g in weight rather than the original ally panel which is 830g. A saving of 34%. The new part is also way more rigid than the ally. I have a post to follow about the part.