Here we have your basic modeling putty. You spread it on with your preferred instrument, wait, and sand. Squadron Green, that horrible Testors stuff, and most hobby shop putties fall into this category. A note for inexperienced modelers - these putties won't dry in thick applications, they're not for structural work, just for filling imperfections and such. And if you fill a plastic model piece with solvent based putty, it will melt. Don't. :)
Bondo Glazing & Spot Putty
This smelly red stuff is my go to putty for seam filling. It's an automotive product, a nitrocellulose (lacquer) based air drying putty. 3M and some other companies make similar putties. Simple to use, squeeze some out, spread it with a toothpick or other tool, wait for it to dry. I use this primarily as a surfacer for larger areas, and for basic seam filling. For me the big advantage of this stuff is that it's the fastest air-drying putty I've used. It also sands very easily, wet sanding with 600 grit paper can cut down pretty heavy buildups quickly. It's also pretty inexpensive and readily available, I can even get it at the grocery!
It does have a few disadvantages. Sometimes it seperates a bit in the tube and I get a squirt of red solvent rather than the putty I'm hoping for. When this happens I use a bamboo skewer to kind of poke around in the tube and mix it back together. It's also probably the weakest of the putties on this list, so not good for use where structural strength is required. Thin with lacquer thinner if needed, though I don't think I've ever had to.
Perfect Plastic Putty
Another air drying putty, this one is pretty new to me. Snow white in color, this appears to be some kind of acrylic based putty, so there's no nasty solvent stench. I expected poor adhesion since it isn't solvent based, but found it actually adheres to plastic as well as any other putty I've used. It's similar in consistency and sandability to Tamiya.
The big advantage here is that it's basically odorless, and can be thinned and cleaned up with water. Water! It's a common technique to use some solvent on a q-tip or similar tool to clean up solvent based putties and reduce the need for sanding. However, this is very tricky when filling seams after painting. Not so with Perfect Plastic Putty! Soak a q-tip in water and it smooths right out! This method is great when working with a seam inside a 90 degree corner, such as a wing root. Also, it's neutral color allows it to be tinted with acrylic paints. When finishing up the dorsal seam on my TOS Enterprise model I tinted this putty with a slightly darkened version of the basecoat, carefully applied the putty, then smoothed it with a wet q-tip. No sanding or painting required! That's a near miracle in my book, so I'm sold on this stuff.
Tamiya Putty White
Another solvent based air drying putty. I've not used Perfect Plastic Putty for long, but it may well turn out to be a good direct replacement for Tamiya. I use this in cases where I might use the Bondo Spot putty but want something that doesn't sand quite as easily, as I sometimes oversand the Bondo stuff and have to apply more putty. It's also a bit less brittle so scribes a little better. I should note here that none of the air dry puttys are particularly easy to scribe neatly, particularly when a line has to cross different materials (styrene and putty for example). Where possible I try to avoid this cross scribing, or at least go with a filler that has the most similar hardness as possible.
The main disadvantage is the drying time, it's really not ideal to sand until the next day. Not great for the impatient modeler, but my favorite hobby specific putty (or was until I found Perfect Plastic Putty). This putty can be thinned with various lacquer thinners. I also like Tamiya Grey fairly well, though something about the little silver flakes in it always make me uncomfortable for no particular reason.
Gunze Sangyo Mr. Surfacer
Mr. Surfacer is awesome! This is essentially a thick lacquer paint, Mr. Color with extra solids. It's something in between a primer and a putty. It comes in various grades, with the higher numbers representing a thinner consistency. I like the 1000 and 500 versions best. This is really good for filling pinholes and pesky seam ghosts that won't go away when using other putties. Have you ever applied and sanded putty, only to find that the damn stuff didn't even go into the recess you were trying to fill? When that happens this is the stuff you need.
I usually apply Mr. Surfacer straight from the bottle with either a small brush or toothpick. If I'm filling pinholes in resin casts I'll typically thin it with acetone or lacquer thinner and brush it on. Sometimes a few applications are needed, which can be done after 20 minutes or so when the first is surface dry. It will kill your sandpaper if you sand it dry, but does sand and feather beautifully if you wet sand. In many applications you can smooth it with thinner on a q-tip and avoid sanding all together. It is available in white as well, though I haven't found that to be a particularly good filler (nice primer though).
Also of note, this is a great tough primer if you thin it. You can thin with lacquer thinner to the consistency of milk and spray it through an airbrush, or brush it on and sand off any bruch marks you end up with.
CA Glue & Cornstarch
I almost put this homebrew filler into the catylized category. It doesn't act much like other air drying fillers, and can set up quickly in very thick layers. It's exceptionally useful! Years ago I kept reading about folks using baking soda to speed the cure of cyanoacrylate (super) glues, and it occured to me to mix it with talc to thicken the glue and make it more sandable. I didn't have any talc, but I remembered that mixing cornstarch with various liquids yields some interesting thick stuff, so I tried it. It works great! Much easier to sand than straight CA, and it cures quickly. It's very hard when cured so great for areas needing some structural strength (exposed corners for example), and for bridging larger gaps.
To make a filler you just need a medium to thick CA glue (I like the purple) and some cornstarch. Make a little pile of cornstarch with a little bowl in the center, squirt in some CA, and mix with a toothpick or something. Depending on the application it may be useful to vary the CA/cornstarch ratio. More cornstarch makes it thicker and easier to sand, as well as causing a faster cure. You don't get a lot of time to spread the stuff before it starts to set up, so mix in small quantities. I usually hit it with some CA accelerator so I can start sanding right away, but it will be ready in 5 minutes or so without. It's pretty tough so wet sand with 300 grit or so to start, but can be polished to a very smooth surface. The mixture does not scribe well though, since it's fairly brittle.
These putties cure via various chemical rections. Two parts are mixed together, then you shape the material and wait for it to set up. I like them because I like to work fast, and some cure very quickly. Epoxy putties are basically clay like and good for sculpting, while polyesters are glop that you spread around then shape after cure. If you need to backfill a part or something, use a 2 part putty. These are exceptionally useful materials, I use them constantly.
Important Note: These products can also be nasty to work with. Polyester has really nasty fumes and epoxy can irritate the skin. I try not to handle epoxy putty much and use rubber gloves when I can, as I've developed a bit of a sensitivity to the stuff over the years and get exzema like symptoms on my fingers if I mix a lot without gloves . PLEASE DO Read the warnings and use caution.
Bondo Body Filler
This is a polyester filler marketed for autobody work. Polyester is so nasty, but Bondo and other "poly-putties" are great tools. You'll see a lot of use in Hobby Japan and such, Mori-Mori is a popular brand. Typically when a modeler says "Bondo" this is the stuff they're talking about. It's available in a few different grades, but I just use the regular stuff. I will point out that autobody guys on the Internet seem to recommend Rage Body Filler over Bondo, it supposedly flows better and sands more easily. I'll give it a shot one of these days. Back to Bondo, it's a grey glop mixed with red catalyst, pink in color after mixing. You can control how quickly it cures by adding more or less catalyst, Bondo is not at all picky about mix ratios. More catalyst yields a quick cure, but you sacrafice some strength. I'd say it's usually about a 10% or 20% catalyst to putty mix, and I'm sanding in 10 or 15 minutes.
Bondo sticks very well to resin, less so to plastic. This can be used to advantage, as in Bondo Squish (article soon). It usually doesn't require extra coaxing (glue) to adhere to styrene, but does sometimes fail to stay put, especially thin feathered edges. I sometimes squirt in a little thin CA when this happens. It sands (wet please) fairly easily, and can be sculpted with knife and files after cure. I use it for a lot of things, most of my models get some Bondo at some point or another!
Aside from the stench, the sometimes big disadvantage is the flexibility of the cured putty. The result is that it's fairly structurally weak in thin sections. You can do a lot of structural work with it, but do be aware of this.
Bondo Professional Glazing & Spot Putty
Despite appearances this is a completely different product than regular Bondo Glazing & Spot Putty. This is a polyester based catylized putty, basically thinned Bondo Body filler. It can be used in much the same way as Bondo Body Filler, and it's lower viscosity makes it work well for regular seam filling and such too.
I believe that it's a little softer after cure than regular Bondo, so not as good for some structural work.
Hardware Store Epoxy Putties
Love 'em! My favorite stuff is Loctite Expoxy Putty (the blue one). I also recently picked up Kwik Wood from J-B Weld which seems to be pretty similar, and perhaps a little smoother. Neither sands to as smooth a finish as the various hobby putties that are available, and they're a little bit porous, but they have one huge advantage - they're quick! These are generally ready to sand in about 10-15 minutes, which is a big improvement over 8-24 hours! I like to work quickly, so these are my go-to epoxy putties. The lack of smoothness is easy to overcome with a good primer, so usually not a hinderance. Plus you can get them at most hardware stores, unlike Milliput and such. It's tough stuff after it sets up, very good for parts requiring structural strength.
This comes as a single stick, from which you cut a slice and knead until mixed. Then you have a clay that you can shape as needed. Be aware that you don't have a long working time, 5 minutes max.
This is the best putty I've found for sculpting, and when I tried it I was surprised at the difference as compared to hardware store putties. The solids in the mixture are very fine, which means it can be smoothed to a very fine finish, and it's not overly porous. The long cure time is a bit of a turn off, but I do get good use out of it. Milliput comes as two seperate sticks, with quite a bit of material in each package.
Tamiya Epoxy Putty
Tamiya makes two grades of epoxy putty, Quick Type (yellowish) and Smooth Surface (white). The Quick Type is not at all quick, it's still something like a 6 hour cure time, so I generally go with the Smooth if I'm using Tamiya. Two seperate sticks are seperated with a thin sheet of plastic, slice off what you need and knead to mix. These are both much more plastic-ey than Milliput or Loctite, so forming them is less like working with clay. This can be nice as they're not as apt to stick to your fingers. In the end I don't use Tamiya a lot, it's on the expensive side and doesn't have properties that are desireable for most of my work.