The foam I meant comes in blocks, its the stuff they use as cores for surf boards. I didn't mean expanding foam as it usually sets with big air bubbles in it. Skin lotion probably wouldn't work either, use hard wax or you can get a spray on release agent. Your going to have to use something like that anyway to break the arch out the mould anyway. Apart from that a mould of your original arch is useless to you anyway as your going to have to cut it out the car to gain tyre clearance Hence why you want flared arches. If you wanted a mould of the original arch the using over it would be the best way to go.
As to how many layers depends on what weight matting you get but your probably wanting somewhere between 1mm and 2mm thick or more if your going full contact
![Laughing :lol:](./images/smilies/hahahaha.gif)
If your not used to working GRP I would recommend you get 300 gram as its reasonably think to go round curves and just put more layers in. The sharper the curve or bend the harder it is to get in. The return on the edge of the arch by the tyre can be a real mare to get right as its so sharp.
I will dig some stuff out for you in the next couple of days. To give you an idea I knew nothing about it but after a little reading I made a mould and from that a new wing for my car. You can judge for your self how it turned out if you have a look at my site. Since then I have made all sorts from GRP and the next thing is to make replica bumpers. Was looking at using carbon kevlar for that but the Airbus 380 has put the price of carbon through the roof so I might just go for a kevlar and glass mix.