Rave',
Magic Sculp, Aves Apoxie Sculp, and Miliput are three examples of two-part epoxy putty. The two parts usually come in different colors that are mixed in equal volume to produce a third color. You will then be able to work with the putty for around half an hour before it begins to set up. When it has solidified completely the putty can be worked with tools pretty much the same as plastic, and accepts most paint finishes very well.
The Testors Contour Putty you're using now is a solvent-based putty. It is similar to Squadron Green and Red putties, as well as those by other manufacturers including Tamiya. These type of putties are carried in a solvent that actually bites into styrene the way plastic cement does; the putties harden when the solvent evaporates. You'll have much less working time with solvent-based putties, but they are more widely available and less expensive than epoxy putties. They can be worked much like hardened epoxy putties, but tend to chip more.
What type of putty should you get? Well, I keep a little of everything, because different putties work better for different jobs. Solvent-based putties work really well for small seams, where I just want a smooth surface and don't have a deep crevice to fill. Epoxy putties are good for larger seams, and can be used to sculpt new features.
Solvent-based putties usually come in tubes, so it's best to squeeze out only what you'll need, then cap the tube - otherwise, the solvent will evaporate out of the tube and you'll end up with half a tube of rock hard, useless putty. The epoxy putties come in sticks or tubes and require you to go to some little effort to measure out equal amounts and ensure that they're mixed thoroughly. You can get a smooth surface before the putty has set up if you smooth it with fingertips or tools moistened with a little water.
Both materials have drawbacks: because part of the volume of solvent-based putty is occupied by the solvent, the putty will shrink and sometimes crack once the solvent has evaporated out. It is best applied in several thin coats, rather than a single thick one. Two part epoxy putties can be more versatile, but also be tricky to use: if you use too little water as a lubricant they may get too dry and stick to you or your tools better than the plastic; too wet, and they lose their adhesiveness. Both kinds of putty will crack if too much stress is put on them. Both should be primed before color paint coats are applied because the putty will look different from the surrounding plastic under paint.
My own preferences are: Squadron Green for small flaws like shallow scratches, sinkholes, etc. and Aves Apoxie Sculpt for deeper gouges and sculpting new details. The Squadron Green handles really well, and Aves is much softer and easier to work than Miliput. But you'll have to experiment in order to find out what works best for you.
That help?
