Final fitting of the tub


So, here is everything laid up as a reminder of where this started. I think it’s a good reminder of how far the tub has come, but at the same time it’s also making me think the duratec is photobombing everything in my garage.




Now, here it is in the car:

It’s pretty close to where I want it now, and I’m doing the final fettling. I made this with a positive mould, which has meant the outer surface (that fits to the chassis) isn’t perfectly flat. I’ve had to take it back a little at a time with the flap disk in the appropriate areas to be sure it fits. I developed a methodology to do this after I’d removed the obvious obstructions.

I go around the gap with a feeler gauge set to 0.7m, and look for areas it traps. When I find a trapped area, I set the gauge to 0.05 mm and see if that still sticks, and anywhere it does, I mark with tape (hence the packing tape on the tub at the back top) and take it back about 1mm with a flap disk. I deliberately only mark the jamming points rather than the tight points so I can avoid making gaps unnecessarily larger than they need to be.

Once everything is marked, I lift the tub out (it’s only 13kg, so two adults can easily lift it with fingertips) and do the sanding. When it goes back in it fits a little better. Everywhere is pretty much where I want it, apart from the back where it’s still a little high. Everytime I advance the fit, the back lowers down a bit. I have about 3-4mm left before it’s flush and I’m happy.

This one is a little more difficult to wrap your head around, but it’s the mating surface between the tub (carbon on the right, starting in the top right corner) and the chassis rail. As you can see, the gap is pretty uniform now, and I’m aiming for between 1 and 2mm.

It unfortunately means I’m doing to have to remove some of the powder coat with a flap disk.




Here is the birds eye view. You can just see the lip at the back. I don’t want to just sand it off – I think I can make the tub a better fir before I have to do that.

Then the final act before bonding it in is to have it lacquered and cured. Then I will have an awesome finished product.

The tub is out of the mould

So, it’s been a while since I posted, but a lot has been achieved, and there are videos below to share the love. So, the tub has been infused, it’s been extracted, the peel ply and infusion mesh have been dragged off the back (that was a sweaty day), it’s been partially fitted and baked.

I’ve got three videos below to show what happened. There’s the infusion setup, followed by the resin going in, and finally the lovely tub once it’s out.

The infusion setup

The actual Infusion going in

The Tub out of the mould – ta-daaaaa

Ready for the bag

 Here is the spiral, with an extra layer of infusion mesh to carry it up on to the part. I’ve also ran my finger through the spiral to separate it a bit.




Spiral is down the middle, jointed by the t-pieces I printed.







Here you can see the spiral as it’s jointed and is taken around corners. It’s much easier to do it this way than bend the spiral through such tight radii.



Here’s the final part ready for the bag. I’m going to envelope bag it this time – I had a lot of struggles with a bag on the flange. I usually do better when I envelope bag it. The blue bits on the top are little cone-hats I made to give the bag some relief from the socket-headed cap screws. They’re in there against a modified t-nut so I can both use air to get the part out, and have a drain-hole afterwards. The red bit on top is one of the two vacuum exhausts. They’re positioned equidistant between the resin feeds. If I need to direct the flow of resin, I have four inputs, one at each corner.

Scrapping Tub 1.0

So, here is the tub out of the mould. I had a lot of issues infusing this, and I’ve learned quite a lot whilst doing it. It’s gutting to scrap it, but it’s the right thing to do. Next time it’ll be right.
There are some high level mistakes that were mine, and they led to some lower level quality issues. It’s the first time I’ve done an infusion of this size and at the same time with a positive mould, rather than the usual negative mould. Doing it on a positive mould means I have to be very careful about the layer thickness otherwise I have a too large or too small gap between the part and the chassis rails I want to bond it to.

Some of the mistakes were down to layup technique, and some were down to getting the infusion wrong.
 I infused over a hexagonal core, and the hexagons were too large to not have a hole put through the middle.
Here’s another picture of the quality issues up close – now for the lessons learned:

Lessons Learned

  • I over engineered the part – too much carbon meant it was too heavy. When I cut it open I had great consolidation – all the issues are cosmetic.
  • In the spirit of making it ultra strong, I had core everywhere. Next time it’s going to be core in the floor, up the back and the top of the tunnel only. The core interfered with the close fit needed to bond it to the chassis. Again, this is due to a male mould. Normally I’d overlay layers in certain areas to achieve this. In this instance, I have to stick to the required layers to achieve a 2.5mm thickness up the sides and at the bonding areas.
  • More holes in the core, and a thinner core
  • The catch-pot imploded. I made one out of plastic. Air got in. A lot of air. I made one out of plastic and tested it for a week – it was strong. With the heat of the part (I had the tub heated to 35C) the pot weakened and imploded. Back to my all steel pot.
  • The tubing sucks up a load of resin. I had 6kg mixed and it flew into the part. This caught me off guard and I didn’t have enough mixed. A small amount of air got in. If I’d had really strong vacuum (i.e. No pot death) I might have recovered.
  • Have one KG of resin and hardener ready to mix. Even two.
  • I took 4 attempts to get the bag on – next time I’m going to make a huge envelope bag instead – it should increase my chances of a perfect vacuum.
  • No gel-coat. It’s a long layup and I was up against the clock. This drove some of the other mistakes. I’ll not be up against a schedule this time.

If I get another output that isn’t cosmetically great, I’ll spray it and be done. If it’s of a cosmetic quality, I’ll have it sprayed in two-part lacquer. This will give a great finish.

Release Agent – pics of the shiny mould

So, the Marbocoat mould-sealer claims to leave the mould more shiny than if it was just ordinarily treated. Well, if you look at the pictures above, you can see that it actually does. Five coats of sealer and five coats of release agent and you can see the result. 

I masked off the mould for about one inch (I know, thinking in old money – don’t care) around the perimeter so there wouldn’t be any issues with the tack-tape struggling for adhesion. As a comparison, masking tape applied to the treated surface just curls off. 
The mould is treated, the templates are made. The modifications to an old compressor to make a vacuum chamber are nearly done (subject of another post soon), and the spray-booth is complete. The next post illustrates just how Dexter I’ve gone.

A study in cores for my tub


So, I need to decide which core to use in the sides of the tub, where I don’t really need huge stiffness like I do with the base, but I still need to do a trade-off between thickness, ease of working, stiffness and weight.

Weight wasn’t going to be my only trade-off here. I have some others to think about

  • Ease of preparation started to become important when I started calculating the time required to prepare a standard foam such as the Airex. It needs to be scored every 20mm as a grid, and then at each intersection, needs to have a hole pit in (only 2mm or so) to allow the resin to flow through to the other side of the core. I did a square metre of it, and it took over two hours. Very tedious to do accurately.
  • Ease of layup is also a significant factor. Some cores are very bendy, some are very rigid, and some will thermoform. I don’t want to choose a core of marginal better physical properties if it takes me days of frustration to get it into the stack. This isn’t just a simple flat sheet, but a large female mould with complex curves in multiple planes.
  • Physical strength






  • do a flex-test to see how rigid it is (clamp one end to a flat surface – hang a standard weight off the other end). You can see how the steel rule is horizontal, and the part has deflected.. 24g is the weight.


  • Weight – Soric (24g), 3D Core (23g), airex (29.5g). So, the airex is 2/5 thicker than the other two, and is 26% heavier.
  • Deflection (therefore infer stiffness) – Soric 30.9mm, 3D Core 31.6mm and airex – 20.5mm

Ease of layup

  1. Soric first – it’s more or less like a thick cloth, and will bend into most angles without being damaged
  2. 3D Core second. It’s a bit like hexagons of foam which are stitched together. It will easily follow simple curves, and can be thermoformed into the tighter ones. It’s not great in tight curves and will separate at the meeting points between the hexagons
  3. Airex last. It has some flex but does thermoform very well. However, getting the hot-air gun in there and pushing and shoving to thermoform it is going to be an arse.


  • I was surprised a bit between the 3D Core and the Soric. Deflections were very similar for very similar weight, which is what one would hope to see. This means the cores were performing as a function of the distance they separate the cloth. I thought the Soric would hold a lot more resin than the 3D core mind, and the results were so close.
  • The airex outperformed for stiffness, which is a symptom of the thickness it separated the cores. Interestingly, i could hear the fibers snap when i weighted this core, and it deformed at the end. Not sure why – further thought is needed.
  • I will use Soric for the side pieces. The ease of putting it into the stack and having it follow tight curves makes it far better for me than the foam. The time and effort to score, hole, form and shape these pieces isn’t worth the extra 600g of weight the chassis will carry as a result.

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.

First template for the core

This is for the side. It’s a complex shape so multiple pieces of card cut-and-shut. Once I’ve transferred this to a 5mm core I will cut the core and heat shape it to the side. 

I’m using solid core rather than flexible in pursuit of minimum weight.