I needed a row of 'Jerry cans' for my T-34, and rather than use all my 'spares' I decided to make my own, casting them in resin. Having read an article in a modelling magazine, I bought some resin & hardener from Halfords (car-parts shop), and some Play-Dough to use as a mold. Well, it is great fun, but do remember to work in a well-ventilated room, otherwise it can become a lot more fun... After a few attempts (including one spectacularly unsuccessful attempt using the white lead stuff scrounged from Space Marine sprues in which I partially melted the plastic dish the mold was in), I finally got a decent set of fuel cans, along with all kinds of other stuff I experimented with. However, I would like to share one thing I've learnt. Play-dough is not the best mold - yes it is nice & soft and you can easily make imprints in it, but it is also springy. Not only is it impossible to get a perfectly flat surface on it, but anything you cast with a flat surface comes out bowed - I'm going to go and get me some plasicine.
For a more permanent mould it may be worth giving a self-hardening putty like Milliput a go. I did try using Plaster of Paris a few years ago to make moulds for casting lead parts and whilst the results were fairly respectable it came as no huge surprise that the moulds deteriorated very quickly. Still if you only need a one sided object casting once or maybe twice it's a fairly good and cheap mould, gives good detail for a few castings and is pretty heat resistant (You have to make sure the Plaster has set and dried entirely, a single patch of wetness could be disasterous and potentially painful). For something like a full figure it is less ideal, and the moulds would probably be more hassle than they were worth. I did also attempt making my own moulds for lead figures quite a while back, but the special rubber didn't cure properly, a lot of the mould came away stuck to the original figure and I eventually gave up on it as a bad job.
