Huge update!
That's right... rust done three different ways so far on this build. The first way is a new product I heard about and decided to give a change. This product is called Rustall. (
visit their website here) Rustall comes as individual bottles or as a set of 4. Bottle 1 is the rust solution, #2 is "blackwash", #3 is a matt coating, and #4 is dirt as seen in the cup below.

Using this stuff is pretty straightforward and simple and it yields some great results, however it's not the end-all solution to everymodel and takes a little practice to start getting the look you desire. There's instructions on a review on thei website, but here's how I've been using it which makes it a little simpler I think.
First pour the dirt contents of Bottle #4 into a dish. Start out with bottle #3, the matt finish. Brush this on the part to be rusted. It goes on nice and thin and levels out beautifully. I really like this stuff! While the part is still tacky, coat it with the dirt in the dish. (the instructions say to do this dirt-dip while your paint is still wet, but my airbrushed paint dries too quick for that) Take a soft brush and brush away the excess dirt.
You'll not notice much of a change in the finish or color with the added dirt, but the dirt is important. It's clay-based and makes for a nice texture and makes the rust solution distribute evenly. Once that's all dry brush on an even coat of bottle #1, the rust solution. It goes on very thin and you won't see much noticible rust on the first try. You add more layers to build-up rust. Without the dirt, the rust solution tends to run like a wash and not "stick" where you want it.
You can add more and more layers of the rust solution, drying in-between but that will leave a fairly even and solid coating of rust. For a more realistic look I brush my second/third coats in random spots and then blend them in with water over the rest of the part. That way the rust builds up in some spots and not consistently over the whole piece. Looks more interesting and realistic this way.
Here's the rust solution added to a shovel. Note the "blade" of the shovel was painted in a neutral gray then sponged with lighter and darker grays thinned with water to give it a mottled, but not textured look. I applied it to the whole blade and handle, the for the 2nd and 3rd apps only around the upper part of the shovel, blending it towards the point with water. The bucket was painted the same way, but no rust was applied yet. This shows the difference. I've found that this looks better if applied over an uneven-colored paint job and not a solid one.

The same technique was done with the Scopedog Visor. The visor was painted in Neutral Gray and mottled with water-thinned darker and lighter grays to give it an uneven, natural appearance. Then some rust colored actylic thinned with water was sponged on in a few key spots. Not doing it all over increases the realism and visual appeal.

Finally I coated it in the matt-finish/dirt-dip then a coat of the rust solution. A few more coatings of the rust solution were applied where I had sponged on the rust acrylic. Finally the part was coated in satin FFA for sealing for later filters and washes. It yealds a very realistic looking fresh rust over steel look though.

The next rust technique is pretty easy as well and yeilds some good results. I totally screwed up when removing the tape from the rust-base coat of this roll-cage and ended up pulling a lot of paint and primer off of the brass dot-plate. AAARRGGHH! I repainted and primed them by hand, but worried that the salt-rust method I showed a few updates ago (method #1 of 3 BTW) would rub the paint and primer off again.
So to do this more gently I tore off a chunk of sponge and dabbed on thinned acrylics in black-brown, rust, orange, and brown. This was really easy and the result is pretty cool. While it doesn't look like the other 2 rust methods I've shown, I think all three look great and show the variety of rust that occurs. If all the rust looked the same that might not look as interesting.

All three of these methods can benefit from being combined. Using the large amount of rust-colored pigments from Mig Productions will also enhance these techniques as I'll show later on when I get to that point. The next two pics of the external tank use several of these methods with good results.
The first rust was shown when I applied the salt over the rust-base coat with hairspray then removed it after painting with water. In a few spots I added more rust with the sponge technique.

The next pic is the aftermath of the first having applied the Rustall system to it. It's a subtle difference but enhances the look. Note that you're looking at the bottom of this hanging tank. The top won't be seen while strapped under the basket on the 'dog's back.

Next is the arm shield which shows the first method of rust I tried, the salt-method. (see a few updates ago for a Step-By-Step) I did this in a faded military green to make it look like an add-on to this refurbished 'dog. Later I'll rub a little graphite powder on the "blade" to make it look as if it's been recently used and that the rust has been worn off.

And here's part of the leg armor, also being rusted and chipped using the salt/hairspray method. With these buff-colored parts I sponged some very thin white-buff color in spots to simulate sun-fading of the paint. This part shows the most extreme example of this, others are more subtle. All of this though will be muted somewhat when filters, discoloration, wash, and pigments are applied later so don't be too shocked about the starkness of it.

And lastly is the insurgent with his base-coats of paint. Looks like crap, yes, but that's fine for this stage and lets me see what it'll look like color-wise later. More opaque oil-paint will be applied over this base-coat rendering it invisible later. The basecoat is important though since the oils are applied thin. Primer-gray would show through a thin white oil-paint for example.
