That is where you are wrong. There have been TWO full sized gundams built in Japan.
Weathering would have to be SUBTLE.
Rust and that sort of weathering would have to be just around the panel and seam lines and be BARLEY noticeable.
Things like dust, dirt, mud, and that kind of grime, well... look at it realistically. Mud and dirt only splashes up so much when something is dropped into it. When putting it on think practically. Where was this Gundam walking, how deep was the mud? Did the foot sink in at all or just splash a puddle? How high would the mud splash. You would never find mud above the knee, ever. Keep this kind of weathering to the feet.
sand is a different story, sandstorms can be massive so sand weathering can easily cover the whole kit, but it is also really easy to overdo it. You would find pockets of sand along places where it can not slide off, ridges, pannel lines, flat parts, joints, etc. Its ok to put a small bit of sand there. Sand would also or atleast could, damage the paint job, so dry brushing is ok but KEEP IT LIGHT!
I have seen dry brushing techniques and paint chipping techniques used on Gundam kits the same way they would be done on a tank and the chip and paint scrape would be the size of the pilot and really unrealistic.
Now, from non natural sources.
Buildings... a Gundam can bump or fall into a building yes, but its the same with mud. How high was the building, where did it bump into it, did it get bumped into it, knocked into it, did the building fall down onto it? All of these should come into play. Also the construction of the building. A glass skyscraper would do different damage then a brick and mortar building,
Weapon damage... Ok, when doing weapon damage you have to think like an enemy pilot. Where would you want to hit this Gundam and with what. So good places to have damage, chest, head, legs, arms. (Yeah that covers everything, but bear with me). Think where the armor is thinnest, like... the knee joint on the leg, or the shoulder. Also, does it have a shield and did it get to use the shield correctly?
It gets complicated when it comes to the damage itself. You can just go and be like ok, the machine gun does this much damage... but if your a fan of the series and want to make it look RIGHT then think about the material. a Gundam is made of Gundamium (lunar steel) and can take a massive pounding and the Zaku MG shells dont penetrate it much. you can put some light putting along the gundam kit to indicate MG shells but dont put massive holes in it. THAT BEING SAID remember the material. The glass visor on the shield, the Camera eyes in the head, joints, all of those are made of weaker material and could break from the Shell.
Melee weapons are a different story all together. I have a few junked weapons that I use specificly for this. Ill get them and strike the kit im building with them with a bit of paint. Where the paint ends up is where I make the damage. Remember for this, is it a glancing blow? Is it a strike with full power behind it? Is it just an accidental hit (mech fell and the beam saber hit the enemy on the way down) Think about those.
A glancing hit with a beam saber would not CUT DEEP into the enemy, it would do some damage, cut in a bit, or just slide over skimming it, think about that before hand.
A full power hit would cut deep but think about the weapon itself and how it was used against it, was it a stab, does it go all the way through? Was it a slash, how much of the weapon hit it, just the tip leaving a small gash or was it the full length of the saber, cutting a large hole in the chest.
Just some of the things to think about. If you have any questions about HOW to do these things ill be happy to answer as well.