Hello, i would like to do a fibreglass bonnet for my car and have a question. It's a simple shape with just 4 lines over the bonnet so i would like to build it on the original bonnet and use it (no mould)

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hi, small remark; I hope you know that if you don't make a mould the part will be larger than the original hood (thickness of the fiberglass thicker) so it could get quit tricky to make 

Hi Alan,

[NOTE: Started writing this before Matthieu posted his comment!]

You do realise that if you use your existing bonnet as a sort of inside-out mould in the way you're describing that the new bonnet will be a few mm bigger than the old one in all directions? This might not sound like much but for a panel that needs to fit back onto the car and line up with the other panels it's a massive difference. If the bonnet is very flat at the edges then it might be OK because you could adjust the panel fit by adjusting the trim lines but if, like most bonnets, it has a reasonable amount of curve the this will be difficult to do.

In answer to your question about the reinforcement, using the biaxial glass would be a good choice anyway but I would try to rotate the alternative layers of reinforcement to create uniform stiffness. If you just use the +/- 45 cloth in its normal alignment then you'll have a panel that's very stiff if you try to twist it but not so stiff front to back or side to side. I would think that 2mm of glass laminate (providing you reinforce it properly from underneath) would be about right. To achieve this you'd be looking at 3-4 layers of a 600g biaxial glass fabric. If you use epoxy resin you'll have something that's quite a bit stronger (for the same weight) or you could possibly drop out a layer of glass, compared to working with polyester resin. Polyester resin is by far your cheapest option though, if you're looking to do this on a budget.

Regarding finishing it; because you're working without a mould, none of your options are great. The finish will be uneven and rough. You'll need to do your best to flat away the texture of the reinforcement and hope for the best in terms of the general unevenness. By the time you've got it looking anywhere near you'll probably have used as much filler, paint, abrasive paper, polishing compound, mops and time to have justified making a mould twice over but I know that making things without a mould is something that everyone decides will save them time and money at some point; even if that's normally not the case. If you do go down this route I think a heavy double spraying of filler/primer, then flatting, filler primer again, flatting, painting and polishing.

I hope this helps,

Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Thanks! 

OK, go for it. Let us know how you get on.

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Well first i have to buy some cloth from you