Hard Point design for the tub

Here’s the design for my hard-point for harness mounting bolts. It’s 3mm thick steel, and the channels are 2x1mm cuts. What you’re looking at is the underside. The channels and holes are to allow resin to flow through and past the hard-point. The big hole in the middle is where the 7/16 insert will go and get welded. Each channel has been tapered down at the entry point so the resin can easily flow into the part.

I’m hunting around now for prices to get them CNC’d. I need 6 per side (six point harnesses) and I’m getting a couple of spare for welding practice.

When they actually go into the part, they will need a layer of glass-cloth either side to insulate them from the carbon to avoid any galvanic issues.

All hail Dolphin Glaze for filling holes

So, in feeling around the part, I’ve found a few small undercuts where I’d repaired the gel-coat with more gel-coat. I went around it with my fingers to look for issues, and found a few where the repair I’d done accidentally still went in a little, like a dimple. I’ve started using Dolphin Glaze as filler – it’s really thin and spreads well. It also comes in a squeezy tube so application is easy. I also bought a pack of onion paper for mixing on – wish I’d done it much sooner.   

It’s styrene based though, but because I’m spraying in a barrier gel-coat first, I’m not concerned. 

So, next is to make a 2×2 frame and apply plastic sheet to keep the dust down, and then I’m making cloth templates.

How to repair mould gel-coat (lots of photos)

So, I’ve been doing the tidy-up on the mould before I infuse it, and I’ve had a bunch of problems with bubbles behind the gel-coat. These are down to a mistake I made whilst building the damn thing. One puts the gel coat on the mould, or it gets the hose again. Well, I put the gel coat into the mould (spraying), and let it set. Then you put a coupling coat of glass on the back. This is pretty thin (tissue) glass – 100gsm chopped strand. Or you do the wrong thing, and try a new product. I tried a coupling veil, which is like breather fabric. I found it was very very hard to wet out, and hard to make it stick to the gel. What’s more, I also started running out of resin. So, half way down the tub I stopped using it and went back to tissue. When you look at the mould now, you can many more bubbles on one side than the other where I swapped from veil back to tissue.

What I’ve found is that I’ve ended up with a lot of repairs to do to the mould. Here’s my technique – it may work for you.

Step 1 – Find out where the hole is

Once you’ve found it, dig it out with a screwdriver – be sure to probe around the edges – it’s quite surprising how far a run is from a simple small blob.




Step 2 – make it concave

One of the things I’ve worked out is that when getting the gel in the hole for the repair, you can’t get it into every pocket a the edge of the gel .If you’re digging out with a screwdriver you won’t get under every edge, and if you’re coming in down from the top with the screwdriver, you may not lever out the weak points as well where adhesion to material and gel isn’t perfect. So, what I do is put a grinding stone on the Dremmel and go around all the edges until the edge of the hole is convex. Then there’s two advantages – one: no overhanging void that the gel can’t get under and leave a small air pocket you’ll be repairing later; two: I’ve found that it occasionally chips out another void that wasn’t visible at all. It only takes a few seconds to do this. Of course kiddies – safety first. This kicks out a lot of dust and is prone to kicking up gel-flakes, so wear a mask and goggles. It’s also loud so i go for ear defenders. I look like the human fly.

Here it is, all dug out. It’s interesting that I found a lot more void when digging it out.


Step 3 – Apply the gel

Gel-coat is weird stuff, and doesn’t fully set in air – it needs a barrier or it remains tacky. There are two ways to do this – first is add a liquid wax solution to the gel coat (typically at 2%). As it sets, the wax migrates to the surface and forms an air-tight seal. It works if you are doing your repair as a one-off or is perfect if you’re spraying a repair. However, I think there are downsides to this for spot repairs:

  1. If you have to build up a repair, you will have to take off the top layer to get rid of the wax (mould cleaner disolves it away, but you’re using expensive chemicals).
  2. If you are building up a repair, it will leave a relatively smooth surface, so not much of a key
  3. The wax is dissolved in styrene (which should totally gas out though) and epoxy and styrene don’t get along well
  4. It is extra faff adding 2% wax to, say, 30g of gel-coat.

So what I do is make up the repair, and seal it with flash tape. It’s specifically designed for resins not to stick to it, as well as being stable at high temperatures. This works quite well for making deeper spot repairs, and the tape gives the gel-coat lots of support if I’m repairing a vertical surface. If you’re using wax, you have to build it up in multiple layers or else it will run. This method is faster (for me).

Step 4 – Flat it off


I don’t have a picture of the large wound I repaired half-way through, so I’m going with this one. If you run your fingers over the repaired gel, quite frequently you can feel it’s slightly proud. If you can feel it, you certainly will see it on a cosmetic part. Worse, if it’s half a millimetre proud or more, it may give you release issues as well. You can see around the repair that I’ve started to flat it off. I find a 120 grade paper on a small random orbital sander such as this Ryobi. It’s great because it also has a extensible pointy nose thing that gets into the corners. Like a wasps ovipositor. Kind of. Maybe. I just like saying ovipositor.
The white speckles you can see are where the gray top coat has been flatted back to the white undercoat. Once you start seeing white, you stop. The white line at the top of the repair is a slight highlight reflecting the light from the spotlight I’m using. It certainly shows there’s a lip there. What’s more, this was two holes so I knew to keep flitting back until I’d seen the two holes again. It takes about 10 mins and some patience to do it with the Ryobi.

I didn’t want to go any more coarse than 120 grit because I’d be digging deeply into the part. 120 makes good progress, but then flats off well. To flat the part, I used an 800 grit disk on a sander like this one which did it quickly and safely . No gouging. From 800 I went straight to 1500, then onto the polishing compounds.


Here’s the finished version of the holes above, or one similar. What you end up seeing is the original holes filled in. once they’re polished (800 -> 1200 -> 1500 polish ->2000 polish -> anti-swirl polish) the new black gel-coat comes out as shiny as the original.






So, here’s the final result, polished as well. You can even see the little loop in the picture in black where i accidentally scratched the gel coat with the Dremmel. it’s in the third photo from the top. This is a very solid repair and will take multiple pulls if necessary





Aramid floor tray is in

Here’s the floor tray, all nicely bonded in.

IMG_3334Here’s the floor-tray all nicely bonded in. You can see on the right of the picture how the floor-tray now replaces the cross-member I removed. I put several extra layers in there as well to pass even more force forward (than the 6 layers of 300gsm that’s already in there).

There’s also extra reinforcement, like a lardy-blokes truss, to take engine mounts if I decide to do that.

So Mark – tell us how you did it

IMG_3316 2


Here’s the part as it came out of the mould – all sharp edges and over-sized. Next job was to put it on the car – mark it up and trim it. That’s the boring bit and I don’t have any photos to document.



IMG_3324This is the chassis before the part is bonded to it. It had been blasted before powder coating, and I used the right high-temp non residue leaving breaker-tape to mask off the mating areas. The blasting heaves a fantastic key. I was worried originally that the tape wouldn’t make a brilliant barrier to rust after coating but it’s worked out fine. The reason being, the coating forms a seal against the tape so there’s nothing to get in – no air or moisture so no rust.

I marked the part up after clamping it down, and drilled for rivets. The rivets were just soft-ally headed ones to provide clamping force rather than to be anything structural.




Here I am as Clampy-McClamp-Face. I had every surface clamped before I drilled rivet-holes. If you drill-rivet as you go along the part, it starts to creep and your holes don’t line up. Clamping the entire thing before drilling keeps the accuracy.



IMG_3325Here’s a close-up of the rivet holes – I’m pretty pleased with how accurately the holes line up. Structurally it’s not really important, but attention to detail matters.








One post-trimming, bonded, riveted part in place. I used an epoxy two-part adhesive. It’s the slightly flexible stuff for parts under a lot of vibration.

Once the adhesive set (24 hours for full cure at 20C) it feels rock-solid.


Air in a composite, under the microscope

So, a while back, I was making a trial part for the tub to understand how much resin a 12 layer infusion would take with a 10mm core. I cocked that up, but since then, I’ve also done an excellent infusion for my floor pan, and I wanted to compare the differences in them both for you under the microscope, because I’m that amazing, informative guy.

Pants Infusion – With Lots of Air in

bubblesIn doing the infusion, I inadvertently admitted some air into the infusion, and it ran over the part. Also, I capped the infusion off once complete rather than letting the pump run for ages to try and pull the air out. Now I know better and know it should be under vacuum until it gels off.

So, what you can see here is a scale at 0.1mm per subdividing line. Ignore the 5mm bit. So, these bubbles are anything between 0.1 and 0.5mm wide. Wherever there’s a bubble, there’s a weakness. To the naked eye, they just look like a very fine dot.

Good Infusion, Where I Got It Right

So, this yellow bitch is going on the car. I had infused the resin at 28 degrees, with the mould also at about 30 degrees. I used a brewing mat under the resin to warm it.

no bubble

This time, you can see no air bubbles, and the weave is easy to see. It’s at the same magnification as the part above, but it’s at a different weave. This is 300gsm twill weave (rather than 2/2 twill) but has less threads per twill than above – more tightly woven if you like. I also let the pump run all night to ensure absolutely no spare resin remained in the part. I also have a new technique to ensure there isn’t any air in the original input pipe, when it’s submerged in the epoxy.

Now, I’ve Keyed it!


In laying up the part, I put some strategically placed 1″ strips of peel-ply in where the chassis rails will be when it’s bonded in. Epoxy doesn’t stick to it peel-ply. When I took the part out and tore off the peel ply, I ended up with a nice keyed surface for the adhesive to the chassis. You can see it here. The crappy red fibers are just bits of the peel-ply I can’t get off. It’s incredibly thin nylon but the red threads are only at the edges. I don’t think they will (at all) compromise the quality of the adhesion.


Resin to cloth ratio by weight

IMG_2088.JPGIMG_2087.JPGSo, here are the two sample pieces, as mentioned previously in this post. They’re rough and ready, and not pretty to look at. What’s more, I had an infusion issue so there’s more air in the part than I would want.

However, it’s worth looking at the following dry weight calculations:

Part Final Weight Dry Weight Diff % Resin
with Veil [1] 136 103 33 24.26%
No Veil 144 110 34 23.61%

So, the lesson is that the part absorbed about 24% resin, excluding that retained in the infusion mesh, pipework, etc. It is also very important to note that the 10mm core is cross-drilled every square inch with a 2mm hole and has resin channels scored in the underside to allow rising to flow over the other side of the core. this will have absorbed some resin as well, which won’t be there if a core isn’t used.

Next post, I’ll get these under the microscope and you can see what the bubbles look like. If I can find a text-book infusion part then I’ll compare against that to see what gets left behind when the job’s done properly.

[1]The veil part is an experiment I ran with a piece of polyester veil under the facing layer to see if it acted as an air-removal medium to make the facing layer more cosmetically pleasing. Due to me cocking up and getting air in the infusion, I have no idea if it would have worked. It certainly didn’t work for me as a backing layer (as advertised). it absorbed far too much resin, was a pig to wet out and didn’t easily go into corners, leaving bubbles behind the gel-coat which have to be repaired.

How much will my tub weigh?

So, based on the sketch I made on the white board (below), I need to calculate just how heavy the tub will be. The main reason I need to calculate this is to be sure the 19kg I’ve taken out by chopping out all that steel and removing the ally panels isn’t then replaced by even more carbon.












I made a trial part, consisting of:

  • 4 * 600gsm carbon (2 above the core, 2 below)
  • 2 * 200gsm e-glass (1 above the core, one below)
  • 1 * 300 gsm aramid (on the bottom, facing the tarmac)
  • 1 * 300 gsm carbon (on the top, to look pretty)
  • 1 * 10mm thick closed cell foam for the core (in the middle)

The layup is symmetrical around the core, apart from the aramid on the bottom and the facing carbon on the top. I have discounted the weight of clear gel-coat applied to the finished part (assume 1kg at the end).

The part measured approximately 103mm * 204mm, and weighed 130g. This meant a unit weight of 0.006 g/mm2.

From this, I fed the dimensions of the panels above into my CAD package. Given a surface extruded to 1mm depth, it will tell me the mass of the panel, to a bazillion decimal places.


Part count Unit Weight Total Weight Running Total
Tunnel Side 2 2.37 4.74 4.74
Tunnel Top 1 1.16 1.16 5.9
Back 2 1.0375 2.075 7.975
Base 2 2.7 5.4 13.375

So, if I go for this, the new tub will weigh 6 kg less than the original steel work.


  • 10mm closed cell foam is used uniformly. This won’t be the case – the sides do not need a 10m core – I will probably go for a 3mm core.
  • the base, back and top of the tunnel need to be strong in bending load, the sides need to be strong in lateral load. As such, I can use a thinner (or even no core) for the sides. I think I will save 1kg there.

I’ve given birth to a mould

IMG_1725.JPGHere’s the tub finally released before it was lifted out. It took quite an effort to release it, with multiple use of wedges, squirting water, and at one point, the 2lb percussive force transduction instrument to knock out the internal bracing in the transmission tunnel out. There was a large amount of plasticine in one part when I was filling in an undercut. At one point I was lying on the ground and trying to push the mould up and out with my feet. There was also some action with the trolley-jack as well to free it up.

IMG_1731.JPG         Here it is out of the tub. It hasn’t been trimmed yet and it didn’t really change colour as I could see after I post-cured it, but it feels as solid as a rock. It’s pretty heavy as well, and I can’t move it around on my own. You can see where you sit, and it’s not symmetrical – it was not designed to be – the Fury has a bend in the transmission tunnel to allow the engine to be offset to the passenger side a little bit to set the weight balance more evenly 50/50 down the centre-line.



IMG_1730.JPGNow we’re looking down it as you sit in it. It is worth noting that the footwells are only in the mould to make it a closed container. I will lay a couple of layers of e-glass in there just to keep the part dimensionally stable, but they will be cut out of the final part. This then gives access to your feet for the CF footwell and steel footwell I have already made. I will need to cast up some jointing strips between the tub and the footwells, just to make extra sure everything passes force to everything else. This process will be a really easy moulding process – just put some gel-coat down,  and then slap some shredded glass putty (like isopon p40)  behind it. I may use glass and epoxy paste though, to ensure better mould compatabity. If I use this, I will need to bake it for a while to be sure all the styrene is out, else I won’t get a good epoxy part.


IMG_1728.JPGHere is the mould now fully out of the car. It’s a positive mould (if you hadn’t guessed) and needs trimming and polishing. There are one or two bubbles behind the coupling coat where the gel-coat will come off, so these need either digging out, or if there’s a small break, in a larger bubble, I can inject repair gel-coat in behind it which will bridge the gap and make a solid plug with the minimum of sanding and polishing. Most of the tub gel-coat is really solid though.


IMG_1726.JPGHere it is from the other side. The white stuff you can see all over the place is the plasticine I used to form the inner radii. I will scrape it off, and then clean it off (hopefully acetone will shift it), then it’s polish, polish, polish.

I have a machine polisher so it’s actually not much of a chore, and there are two coats of gel down, so if I start seeing white behind the gray, I know I’ve gone as far as I dare.



IMG_1732.JPGHere we have an inner radius with the plasticine scraped off. There’s a small ridge there (looks a lot worse in the photo) which will sand off with a bit of wet-and-dry. I will start with a fine grade (say 800) and see how that does, before finishing off with 1200, then 1500, then polish, then wax.





IMG_1729.JPGFinally, here’s the money shot down the tunnel.

The inner radius transferred brilliantly from the part, but it will need some rubbing to bring up to a good polish. I did form the shape with body-filler and then waxed it, so it was always going to be an OK finish. Because I sprayed PVA release over the poly board (which is meant to have a good inherent release), I didn’t get the full shine from it, but there is a bit of a shine there already. It will be trivial to polish up.




Look at the bloody mess the demoulding process made. This was after I gave the garage a bit of a tidy and did a tip run. Sigh. Guess I’ve got to do it all again.