wrongness 6 – angle grinder accident

wrongness 6 – angle grinder dies

So, after 20 years of faithful service, my bosch angle-grinder only goes and dies. Smoke came out of it and that was the last turn it made. I replaced it with a cheap hitachi and kept on going. There’s a lot of steel seam-welded in this car and it can be a real pain to get it all grinded out. Near the end of the day, I was starting to feel like I’d had enough, but was wanting to get one corner finished.

Therein lies the rub :). I was wearing a jumper and managed to get the left sleeve of my jumper caught in the grinder (I think I was holding it in my right). Before I knew it, it had whipped the grinder out of my hand, and as it started to spin on the sleeve, it dragged the grinding disk up my arm. Then it stalled.

I don’t like thinking about what it would be like if I had a slitting disk in there rather than a grinding disk.

Here’s the obligatory photo though:


An infusion went wrong

So, I was finally ready to infuse the transmission tunnel top, after deciding on a compromise. The compromise was to not spray the part with clear gel-coat first, even though I had some scratches in the mould. [1]. I reasoned that the scratches will leave a positive on the part, and that can be flatted off with some wet-and-dry and a little polishing.

IMG_0137.JPGSo, I laid up the part (one layer of 350 facing cloth, and one layer of 200 backing), found to my surprise that I got the bag to seal first time, and set about infusing the part.


This is the first time I’ve had a stuck infusion, and as I think about it, it was a culmination of a bunch of factors all adding up together to cause issues. The factors were:


  • IMG_0138.JPGI decided to infuse along the short side and go up the full length of the part, rather than infuse across the shortest distance (I’m running low on infusion spiral)
  • it was 10C in the shed when I was mixing the resin, and I thought “It’ll be OK – it means I can use quick catalyst and it won’t go off all that fast”. I ignored the gloopy sensations.
  • I was feeling impatient, and thought I could get away with quick resin, allowing 10 mins for degassing because it was cold
  • my oven is out of action: it’s in pieces while I make it bigger for the transmission tunnel. I thought I had enough kingspan for the job, and I was short by one roof baton length. This is now bought and sitting in the boot of my car. This again was an incentive to go for the quick catalyst
  • when I was mixing it, it felt more viscous than normal, which should have been a warning that stuff was about to go wrong.
  • when I infused it, it got stuck half way along the part and the resin went off in the pot.


So, I think the resin being too viscous meant it didn’t march along the part quick enough before it started to go off. If I’d used slow resin the infusion could have ran at the snails pace it was going at and it wouldn’t have been a problem. What’s more, I could have warmed the resin up (with the slow catalyst) and it still wouldn’t have been a problem. For the handbrake bracket, I actually had the resin at 40C to ensure it wetted the part out properly. So, this evening I will finish the oven, take the half-part out of the mould, salvage the cloth I can which I’ll keep for backing layers, and have another crack at it.

[1] My compressor has died, and the replacement part (£160 if I can wait for a machine-mart VAT free weekend)


Mould Making, and issues

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.

photo copyThe 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.

photoand … Disaster




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.

Lessons Learned

  • 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.