We´ll start with a simple "rusted-paint" material (this will be VERY simple, it won´t look too good. It´s simply for explanation how the method works.. try to understand how max creates materials and then simply... gues what? ... be creative! ;-) ).
First go to your material-editor. (in max, press m)
Get a new material. Create a plain "raytrace" paint material, something like shown in PIC12 (no, I´m not going to explain that process, that´s very simple. All you need to know is included in the tutorials that come with max). Name it "Paint" (YES, name your materials unless you want to become mad.)

As you see, it´s very simple. Just a shiny, glossy material with fall-off in the reflect-channel /set to fresnel) and, just for the nice look, a hdri-image in the environment-slot. We will remove that later as soon as we render in a proper environment. If you want to use hdri-files: Go to www.splutterfish.com and get the free hdri-plugin from the download-section! Thanks to splutterfish for that....

Now we need a rust-material, don´t we?
Here it comes:
Make a new material, "standard" is ok, name it "rust". Chose the diffuse-channel - add a supernoise-map (yeah, get it now if you haven´t got it already ;-) ).
Change the two colours to dark-brown and a medium brown. This will be the main colour of the rust later on. Then click on the "supernoise"-button.
See PIC13 for details.

Leave all the other paramters like they are for now.
From the menu chose "mix" now, chose "keep old map as sub-map". That allows you to keep your supernoise map as one of the two sub-maps in the mix-map that you´ve just created! Then drag your old supernoise-map into the second slot and chose "copy". Tweak the colours a little bit to get more randomness, also change some of the main paramters like size etc. (I can hear some of you scream "Idiot! Why don´t you explain that better?!" --> Because it´s not important for now! =o) This is only going to be a demonstration, not "the" real material. The parameters always depend on the size of your object so you have to TRY which parameters work well for you.)

But for all of you who are really nosy: See PIC14 for the values I chose ;o)

Alright. Go back to the Mix-Map and chose 50 (percent) for the mix-value for now. You might tweak that later on if you like.
Name the Mix-Map "rust-colour".
Now go back to the root-layer, from there on go to the bump-channel. For the bump-map choose the map you´ve just called "rust-colour". Choose "instance". So you´re using one map in two channels - that gives a much more "realistic" look later on. And you can change the look of your material by changing ONE map.For the bump-value chose something like -50. (that means, the rust is "deeper" than the paint. Well, not really, but the rust will look more like it has grown into the paint) SEE PIC15.

Now we come to the interesting part!
Choose a new material, make it a "blend" material. Put "paint" in channel one, put "rust" in channel two. For the mask choose a new supernoise-map.
Adjust those values of the new supernoise until it looks good to you (remember? There´s no "right" way! Try it! You won´t learn it if you only copy my values..). Now your material should look something like this tea-pot in PIC16...

Not too good? Right! It still looks very crappy. (Does this word exist in english? Sounds funny in German though!)

But did you understand the basics? You mix different maps until you achieve the look you want. From here on, simply add new mix-maps to your materials, especially (!) the mask-channels of your blend-materials. Please go and get the electricity-plugin from blur as well as this will provide you with really nice irregular maps. Also get the "scratch"-map.