I use Milliput a lot. Not just as filler, but also as sculpting clay. At least half an hour's work-time ... and a day's curing time. I have not found that much difference between the three different grains.
The two parts are kneaded together - if you don't use gloves, be sure to wash under the nails afterwards.
I seldom need to mix a pea's size of Milliput at once... so I buy maybe at most one pack per year.
It can be smoothened with water. Water makes it also less adhesive, and you can imprint textures on Milliput if it's surface is wet. You can fill multiple pinholes and scratches on a prop by rubbing a small piece of it over the prop's surface with water.
I use glazing putty mostly for small pinhole cracks ... and when I am impatient. It dried very fast, but is sticky when you use it and is not very durable. Only very thin layers. There are several brands that are more or less equivalent: Tamiya, Squadron and Italieri are those I have used. It smells a bit like spray paint.
Auto Body Fillers such as Bondo are useful mostly for filling on quite large props, such as helmets. It is must softer than Milliput so it can be spread out. Most brands cure pretty fast, but there are exceptions. They are based on two-part Polyester resin, which smells a lot, even after curing and you will need proper ventilation or be outside when you use it. You actually should use an industrial breathing mask when sanding it, but most people use dust masks...
"Rondo" is Polyester resin + Bondo, to make it softer and have longer curing time.
I have transitioned from polyester-fillers to mixing my own epoxy-based substitute that doesn't smell, is more flexible and has more adhesion. I mix regular two-part epoxy glue or laminating epoxy with a couple of filler-powders: most importantly Glass Microballoons which is a light filler and Colloidal Silica to prevent it from running. Microfibers can add strength. I need to take extra precautions so that I don't inhale any powder when I mix it, but most of all it is it is more expensive. It does not sand as well as Bondo if I used too much epoxy.
I suppose that I could make it have the properties of Milliput if I get the mix right, but it is much easier to just use Milliput.
