I second what [MENTION=6588]pogre[/MENTION] said. If you are not planning on doing this for "professional" casting on a high pressure vulcanizer then your options are limitless. As a matter of fact it was pogre's sculpting challenge a few years back that really started me on this awesome madness.
Each sculpting material will behave differently so you have to get used to it or you will get frustrated.
I've used sculpey, super sculpey, Kneadtite Blue/Yellow (green stuff), milliput, and ProCreate. It has all depended on what I wanted to sculpt.
I will usually use sculpey, and/or super sculpey for "larger" pieces. Also for pieces where the fine detail is not that important to me. Both are oven baked clays. Sculpey feels like clay when you're working with it and super sculpey feels like soft play-doh. They are so similar I'll talk about them as one.
It is cheaper by volume than the 2 part epoxies and holds rather well. It remains soft until baked so it is easy to "repair" mistakes. But because it remains soft it is also frustrating at times - you can make more mistakes on parts you had "finished". Both materials can be smoothed over quickly with a damp rag making them very easy to work with. The material is light weight and remains somewhat fragile, even after baking it. Don't drop after baking, or you will probably have to glue shattered pieces. Sculpey remains slightly more porous than super sculpey after baking.
I just started using ProCreate recently. It is a two part epoxy that hardens solid as it cures. The box it comes in has important mixing ratios (hardener to resin) to determine cure times (hardening). I have found that it hardens significantly faster than the other epoxies I've used. It has also frustrated me a bit because it has a tendency not to stick to the armatures I'm using. However, I'm still getting used to it so a lot of that could be "user error". Because it cures faster, I can start working on a piece faster, but I have a lot less time to work with it. It holds details well, and I find the color easier on my eyes to pick out detail as I'm sculpting. I'm going to get a new batch of it to see if the curing/sticking problems I'm having has more to do with the specific material I'm working with. Even when cured it has some flexibility to it. I've been able to do slight bends to the material once cured. As long as I was using a wire armature underneath the bend would remain.
Milliput, is also a two part compound. I've only used it once and it was a long time ago. The type I used was the white/gray. This particular one smooths over very easily with a damp rag/cotton ball. The reason I stopped using it was because when cured it was too porous for my taste. However, I have a friend that uses the white/blue compound and he swears by it. What I love best about milliput is that it sands beautifully.
Kneadtite (green stuff) is also a two part epoxy that hardens solid as it cures. I have found cure time for it to be a bit longer, so it gives me more time to work on something. However, because of the longer cure time I can put my fingerprints on it a lot longer too - frustrating. When fully cured it retains some flexibility (bends). Of all the materials I like this one the most, but that is because of my familiarity with it, not because it is objectively superior.
Kneadtite (brown stuff) is another hardening two part epoxy. However, this one will cure solid, and totally stiff (no bend). It is great for making weapons and very sharp details. This one also sands beautifully.
Sculpting is time consuming, it will expand your expletive vocabulary at times, and the materials for sculpting are not necessarily cheap. So don't undertake this to "save some money" on miniatures. I find sculpting very fun, and that is why I do it. It is also awesome to plunk on the table a monster that my players have never seen as a miniature.
I would recommend to start small and get very familiar with the sculpting material. If this is your first sculpt of a miniature and you are on a tight schedule I would recommend finding a miniature that is similar, and doing some conversion work to get it to what you want. It will probably be less time consuming to begin with.
When the sculpting putty is not doing what you want it to do, it is the most time consuming and "frustrating" part of the "pushing putty" process. That is why it's so important to get used to the material. Have your friend look at as many sculpting tutorials and books as possible. 4 weeks is not a long time if they have never sculpted before.
Most important of all, practice, practice, practice, and don't quit. Your first results are probably going to look primitive. Don't fret, just keep doing it and with practice you will see your efforts improve.
I have posted on my blog some of my sculpting efforts.
Figmentia attacks - Insomnia Parries - Blogs - Loremaster
More Insomnia - More Sculpting - Blogs - Loremaster
Good luck with your sculpting. It is fun and totally worth it.
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