Hi Paul...you can thin milliput and Magic Sculp with water to 'paint' on detail. Magic Sculp is better for this as it's not grainy like Milliput, but you can achieve much the same results with either.
Once diluted or liquified, keep in mind that the carrier (water in this case) will gradually dry out as the Magic Sculp cures, so what you see as detail when initially applied wet may not be 'exactly' what you get when final cure is achieved. It will also be somewhat delicate, much more so if you use Milliput. Magic Sculp is far more resilient under such usage.
Naturally, as with any technique you try for the first time, a little practice will be needed to get the desired results. The only way is to experiment and keep practising. Timing is also important as Magic sculp goes through several distinctly different stages while curing and you may find that as the diluted mixture sets-up a little it will become more useable.
Hope this gives you something to build on.
All the best...Roy.