To start with, you paint your model with what you want the "weathered" color to be (a rust color, in my case):
I used a can of Krylon "Ruddy Brown" primer - I think it was around $4 at Wal-Mart. The label says it's for "Metal - Wood - Wicker - More". Unfortunately it doesn't say anything specifically about whether or not it's safe for plastic. I've never had a problem with Krylon in the past, but to be sure, I grabbed a model that I wasn't particularly attached to (a Warbuggy I got in a Craigslist lot):
After spraying it & waiting for it to dry, I didn't notice any adverse effects - so I grabbed the rest of my models and let 'er rip:
I sprayed them from several angles to get good coverage:
After they dried, I was ready for the next step:
This was the cheapest can of hairspray I could find - I actually had to dig around to find a can that wasn't four dollars or more. I finally found this one at the store on the very bottom shelf, in the corner, for 97 cents.
Then I grabbed some salt:
This was free, 'cause I grabbed it out of our pantry. IMPORTANT: YOU MUST USE KOSHER SALT FOR THIS TECHNIQUE. Regular table salt (AKA "iodized" salt) will not work. Why? Well, if you look at "iodized" salt under a magnifying glass, it looks like this:
...it's almost a perfect cube. The shape of the salt is going to determine the pattern of the weathering - unless you live in a world where rust spots form in perfect geometric squares, it's gonna look terrible if you use iodized salt.
This is what "kosher" salt looks like:
...it's irregular, random-sized, broken pieces (this is due to how it's manufactured). That's what we want.
I dumped some of the salt into a bowl to make it easier to work with:
Then I sprayed my models with the hairspray:
...and sprinkled the salt over them:
I worked in sections - I would do the tops of the models first, then after that dried, I did the sides, etc.
Then I gave the models a coat of GW Chaos Black:
...about this time, I heard something going on outside my patio: lawnmowers! Crap! The gardeners for my complex had come to mow the grass - threatening to get grass clippings all over my wet models! I stood by until I was pretty sure the first coat of black had dried, then I rounded up the models and stashed them in my kitchen:
Later that afternoon after the gardeners had left, I took the models outside and finished spraying them black. Again, I sprayed them from several angles to get complete coverage:
After the paint had completely dried, I took the models inside, held them under lukewarm water, and scrubbed the salt off with an old toothbrush:
It will take a while for the salt to come off - it seems to be easier once the model gets thoroughly "saturated" with water. (I have a suspicion that submerging the whole model in a bucket of lukewarm water would be effective, but I wasn't willing to try it).
Here's the result after the salt is scrubbed off:
(click to embiggen)
...you can see that the rust-colored paint shows through where the salt was covering the model, leaving a "flaky rust" effect.
Some random thoughts on the process:
-It uses a lot of paint - I used two cans of primer & two cans of Chaos Black (of course, I did five Battlewagons, three Trukks, five bikers, three Deffkoptas & a Warbuggy).
-Getting all the salt off can be a pain - I scrubbed until I thought it was all off, let the model air dry, then had to go back again to get some spots I missed, and to get rid of the white, salty "film" that was left in some spots.
I wish I had used more salt - I read through several tutorials on the Internet beforehand, and a common mistake on the first attempt was using way too much salt, so I used it sparingly. Unfortunately, I think I underestimated how much salt fell off while I was turning the model during painting, as well as their unscheduled trip back into the house, even with the hairspray holding it on. There was one model where I was afraid that I used way too much salt, and it came out the best out of all of them.
-This is not a "one-step" solution to weathering; the model is going to need some traditional weathering done before I wam completely happy with it.
-This method can be kind of rough on your models - three or four of my vehicles needs parts re-attached that broke off in the scrubbing.
So would I do it again? Probably, although this method is usually presented as a time-saving method, and if you're doing seventeen vehicles at once, it still takes a lot of time - pretty much my entire day off...