This is a long overdue post to expound the wonders of Milliput putty. I've been using this stuff pretty much since I started modeling and converting miniatures way back when I was a wee lad. Let me start by makign it clear that I don't do a lot of sculpting - beyound basic items like extending hairlines or adding chainmail - but I do use this putty for gap filling and base building. In this respect I can't praise it enough for its durability and versitility. Milliput works by mixing equal parts of the two different colour putty's together for about 4-5 minutes depending on the size your using. I tend to employ a ‘folding’ technique to avoid streaking and uneven mixing of the putty. Basically squish two balls of putty together into a flattened oblong. Then fold in half and squeeze flat again before folding again. Continue this process for a few minutes or until you have a uniform colour.
Ensure surfaces are free of dirt or grease before applying the putty. Surfaces can be cleaned with solvents or soapy water. I use a soft bristled toothbrush and washing-up liquid to wash surfaces before painting or sculpting. Most metal miniatures may still have mould release still on them or grease from handling so its a good idea to clean models before working with them even if your not using putty.
Freshly mixed Milliput can be very sticky so I dip fingers and tools in water before handling and working with it. However you can wait and let the mix cure slightly and work with it after an hour or so when it becomes less tacky and more rubbery. This is fine for sculpting but of course you miss out on the best adhesive properties when you use part cured putty. Always mix a little more putty than you need and keep the excess after you have finished sculpting or filling gaps. This extra piece serves as a way of testing the strength of the cure as it will set at the same rate as the material you used in your project. Once cured the beauty of this material is that it can be drilled, filed and even sanded while still retaining its adhesive properties and bonding strength.
Mixed Milliput can be stored for up to 36 hours by freezing it. Freezing slows the curing process and means a batch of putty can be made and stored to be used over a couple of days. Officially the shelf life of Milliput is 2 years but I have been successfully using one particular pack that I know for a fact is over 10 years old. Shelf life can be maximised by keeping the unmixed putty in airtight bags.
Lastly its worth mentioning Greenstuff and other custom epoxy putty's for the miniatures market. I have used various brands but always come back to Milliput.