I used gun metal and exhaust bottled colors, not the spray cans. I used a cheap Harbor Freight airbrush to apply over the black. You can also use the exhaust buffing metalizer also. It's a bit more metallic than regular exhaust. I also got the metallizer sealer to put as a top coat. It keeps the metallic from rubbing off and gives a nice flat sheen to everything. I just use the exhaust and gun metal on different parts to give the parts a little individuality so it doesn't look like everything was sprayed with the same finish.
The Stainless Steel will come out brighter so you can still use it for parts that should look a bit brighter.
I also mixed in some brass metallizer to the stainless to make the "silvery" parts a warmer tone - like the ammo chute. I compared the stainless steel to the original ammo chute links and the chute was a warmer silver, not as blueish. So, I mixed in some brass into the SS.
Did I buy the correct ones in your opinion? I looked at your photos again and they are quite darker than the colors of the metalizers....I am guessing that laying the flat black down may help to get it like your color.
I used Testors Model Master Metalizer paints. They're lacquer based paints in the small jars. You can find them at most hobby stores. They give that metallic look without being too metallic. It does a good job mimicking a parkerized finish.
I put down a flat or satin black or dark grey, then paint over it with the metallizer.
"Could you estimate how much this process adds to the parts' skin thickness?"
I use an automotive spray putty to get fine finishes. the layer thickness is only a few microns, but to get a good result you do need several layers and light sanding in between. The final layer is sanded with a 800grit wet and dry and it comes up smooth and shiny! Throw a couple of layers of gloss automotive paint on this and the result is beautiful.
As long as you allow for some clearance when designing parts that fit together (as you would anyway) its never been a major problem, even along a long curved mating surface as in this image. Some sanding will always be needed to make up for small deviations and inaccuracies, even without primer or putty.
I'm not quite sure on that. I'm sure it varies over the surface. If I have any mating surfaces or parts that need fitting, I will sand to fit. It's more or less building the surface enough to sand back down to the layers ridges so you're basically filling the valleys. PLA really sucks to sand. I know ABS would be easier but have not setup my printer for ABS yet.
Amazing results! (I sound like a TV sales guy..."Now how much would you pay?")
Do you find since you are building up the part a little that you need to be more careful of tolerances of mating parts? Could you estimate how much this process adds to the parts' skin thickness?
Here are a few parts that I finished. They came out great and didn't have to sand the PLA. The trick is to use a high build filler primer (Duplicolor) put on about 3 to 4 coats and sand down to the ridges. Works great and you don't have the nightmare of sanding raw PLA. In pic "painted2.jpg" the undersides are not filled, just painted. For the top coat I used acrylic model paint shot through an airbrush.
I wet sand the primer with 400 wet dry paper. Dry sanding will not work well, the paper will load up very fast. Also attached a pic of the Duplicolor primer that works great!