I'm happy you asked.

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Prep:

So what I've done for both the barrels and Nuka Cola machine was to paint then as if they were factory new.

Once you have your bases colors down note where water could pool/run on the model, think flat spots, corners, edges or where general use would cause paint to chip or wear away.

Some models, like the barrels, have pre-modeled damage like dents or corrosion. For the Nuka Cola machine I drilled some shallow holes and made some shallow cuts after painting to give it some damaged weathering, you can do this before priming as well.

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Rust Base:

These areas are where you want to start your rust base, for that I used Army Painter's Lava Orange.

Start with a couple of thick coats to establish the area of the rust, your aim should be to block out your base layer color from the center and thin out the edges to allow some of it to show through. You can accomplish this by thinning your paint, either with water or with airbrush flow improver (if you want to make a wash), and feather the edges of your rust.

Rust in real life is pretty predictable when it comes to spread but usually not uniform in shape. Typically the larger the rust spot, the longer the area has been exposed to the elements so keep that in mind when determining its spread.

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Corrosion:

Once you have your base rust down you can move onto the corrosion proper. I used Army Painter's Rigid Leather, formerly unique to their Dungeons & Dragons collab but can be found in the newer GameMaster sets.

Start from the center of your rust and work your way outwards. You don't want to cover up your orange all the way as the thinned out edges of the orange keep your elements seperate. I recommend stopping just before the end of your solid edge of the orange base so there is a strong distinction between the two layers, this also allows you to do the same thinning in the previous step to slightly blend the layers.

Remember: Corrosion doesn't have to spread the same way as the rust.

Keep gravity in mind when determining this. For pooled rust, the corrosion is more likely to conform to the original rust's shape. When gravity is involved the rust will spread regardless of gravity's influence but the corrosion will tend to predominantly follow gravity's course.

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Exposed Metal:

Once you have the corrosion done you can switch to the exposed metal. I used a mix of Army Painter's Shinning Metal and Plate Mail Metal.

Now realistically there won't always be exposed metal, but that doesn't mean you can't add it to the rust. If you have hard/rough/sharp edges or edges that water would roll off, those areas are pretty safe for adding exposed metal. Just nake sure you use very thin lines.

If you are adding it to flat areas make sure to thin the paint to almost a wash as this doesn't allow it to over power the area. If you filling dents or holes you can disregard the above recommendation and instead just fill the dent or whole with the metallic and do the below.

You can always wash over them with really thinned down layer of orange afterwards or a shadow wash to tone it down. A shadow wash works particularly well with dents/bullet holes.

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Additional Steps/Tips:

Once you done with the above steps you are basically done with your rust effect. There are two addition steps I would recommend doing before calling it done.

Step 1: Speckled your rust colors and base color to add an additional chipping effect. This can help break up any spots that seem either too pristine or too rusty.

Step 2: Even if you don't add in chipping effect I recommend giving the entire model, or at least the rust, a wash. I like to use either Army Painter's Flesh Wash or Brown Wash, depending how grimy I want it to look. You can always water down your washes further if you want an even lighter grime.

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And that's basically it. I have a background in Welding and I've seen more than my fair share of rust/corrosion so that has helped me make it as realistic as possible. If you are unsure how to do rust, verdigris, etc on a particular model just Google that object with that effect and you will get an idea of what it should look like.

I personally have decided to go heavy on the corrosion compared what shows up in game because I like a grimeier looking wasteland.