My general Bondo workflow, bare surface to beginning of surface prep. General requirements: Bondo (obv), workpiece (obv), Bondo glazing putty, mixing surface (I use yoghurt/dip lids) and associated mixing tools, respirator, nitrile gloves, assorted files, wire brush for cleaning out the files, 60 grit emery cloth, wet'n'dry paper from 220 grit upwards, sharp knife, etc.
Work above 50F, preferably around 70-80F. Make sure you have an applicator that has a flat edge. Multiple applicators of various sizes are handy, I use around 1/8" to about 1.5". The scrapers designed for auto body repair aren't very useful for modelling work since they're huge. Wear a respirator at all times! When the container is open, during mixing and application, and during shaping. The former needs an organic vapour cartridge, the latter a particulate cartridge. Usually the all-purpose cartridges the MSA/3M respirators come with can do both plus ammonia/amines and some other solvents (magenta/olive NIOSH labelling in the US). Definitely worth the 30 bucks. You should also wear nitrile gloves since the hardener is pretty nasty, though I'm frequently guilty of not doing this.
1) Bondo's pretty lax with mix ratios. You're aiming for about 2-5% hardener, which is a pastel salmon colour. You can use up to 50:50, but then it'll be very rubbery and a terrible waste of hardener. Sometimes you want that rubbery though because it's easy to carve with a knife once it's cured, but only on very small details.
2) First mix up a small batch and scrape it thinly over your current section of work area (probably no more than a 4" square at a time) ; the action of spreading forces the resin into any pores (porous materials) or scratches (roughed up non-porous materials) in the material and provides a well-bonded surface to begin with.
3) Once it goes bitty and crumbly and doesn't really want to stick, stop trying to spread it. It'll just crumble back off so it's a waste of time. The only time you can use this stuff is for literal hole-filling of 1/4"+ diameter, 1/2"+ depth holes or channels, where you'll be coming back with another layer to seal it in.
4) Once it sets up and goes rubbery (generally 3-6 minutes from mixing depending on hardener ratio, temperature and layer thickness), trim off any excess with an Xacto. Don't worry about the little peaks and troughs on your surface, they'll provide mechanical grip.
5) Start measuring out your main quantity of Bondo. You're aiming for about a 1/4" maximum thickness over your work area section, typically, though some uses you can get away with really glopping it on. If you're filling in an area with walls, you can use the walls as rails to set the height with the scraper.
6) Now comes the most important part. First cut your Bondo to size. An Xacto is fine for small details, for bigger stuff you'll need a boxcutter or snap-off utility knife. You can also sculpt it right now, which is good for making bevels and other hard-edge geometry. Sculpting smooth surfaces is a waste of time and blade sharpness, just ignore that for now. You have about 5-10 minutes of sculpting time, a little more if you're using a stronger knife!
7) After 30 minutes or so it'll have finished the bulk of the curing and it'll be hard, no longer rubbery. Careful, since now it's also quite brittle. Now you need a coarse file or fine rasp. Don't press very hard at first, just knock down the lumps and bumps until you get a feel for how much material it's taking off (a lot!). Once you get the hang of it, you can go to town on it and eliminate huge sections of overfill or shape erratic blobs into gorgeous smooth curves. Remember not to use a "sawing" motion with the file on curves; get your hand and wrist curving with it it to ensure no flats (which are a pain to fill back out). You can also use 60 grit emery cloth to magnificent effect here.
8) Take a medium file or some 220 sandpaper over it now to smooth out the planes you have. For fine details, use needle files. Once you've done this, you can see all the shiny little pits and voids from air bubbles that are inherent to the process.
9) Mix up a batch similar to your first skim coat, and scrape/smooth it over your surface. Pay attention to all the voids that you previously saw, and try to fill them all in without adding too much bulk to the surface you just so carefully sculpted.
10) Don't bother with the trimming this time, except in cases of overhang. Trying to carve your skim coat will most likely just result in you ripping it up and having to start again. Just wait for it to harden up completely (maybe an hour) and work on something else or a different section for now.
11) Grab your coarse file and knock down all the lumps and bumps of the skim coat, then true it all up and blend it together with your medium grit. You should, in theory, have a pretty close to perfect surface now, though sometimes this last skim coat needs doing twice. Patience is rewarded in this process.
12) Once you're happy with the surface, sand it thoroughly to 320 grit. This will start to put a slight sheen on it, and grind through any minor dimples you might have missed. It'll also, once you've blown all the dust away, reveal any pin holes that need filling before you can proceed to surface prep and finishing.
13) Grab a nitrile glove and small popsicle stick broken in half or something (I use MDF scraps off the floor). Fill the pin holes with Bondo air-drying Glazing and Spot putty (or a similar glazing putty, there are plenty out there). Now use your finger and the little tool to smear and dab tiny amounts of putty into the pin holes.
14) Sand any little lumps of putty down with the 320 grit sandpaper. Once your entire helmet/gun/plate/whatever is to this stage, you can probably wet sand the whole thing at this point.
Now you're ready to either continue up the grits, wet sanding as you go (I'd recommend at least 400 grit before painting), or lay down some filler primer and see how you're looking. That next part is up to you. If you want some visual cues, there's a whole bunch of this stuff at various stages in my BR85 build thread. Not a helmet, but Bondo looks the same.
Thanks! Since it seemed reasonably well fleshed-out after finishing it, I decided to edit it a little and illustrate it as a blog post: http://robtaylorcase.com/blog/?p=399You pretty much covered everything. very good description in every step. Fellow 405thmember will appreciate it, and thank you for taking the time to post this.