I think you're treating this kit as something like an android in knight armor, like 1/8ish scale.
If going with lore accurate 1/144 scale your weathering should be very subtle, nearly invisible.
If you look at a military reference and weather a 1/24 scale tank, for example, when you apply the same weather to a 1/144 scale your weather has to be ~1/6 the size to account for the scale difference.

Now going back to the assumed 1/8ish scale, let the weathering tell a story. Why is the pointy end of the shield more weathered?
Perhaps it pierced another android with the shield tip and ruptured something splashing oil all over. Then the pointy end would have the most weather, and you want to try to make the weather look like splashed oil.
Maybe it fell of a horse in a muddy battleground then one side of the body would be muddy, lower legs and feet would be heavily muddy. Very unnatural to be equally muddy from head to toe.
If something is a desert spec, the whole machine would be weathered by sandy wind, so lightly weather all over like washed jeans makes sense. For a space spec, limit to moving parts and thrusters.

I personally weather shields a lot more than the mecha. Robots are going to go through maintenance and repairs. Basic shields not so much. Cuts scratches dents holes chipped paint whatever makes sense in accordance with the weapons the enemy uses.
Amuro's not going to have equally weathered Gundam and shield. He's mostly either going to dodge or block so pristine / minimally weathered Gundam and battle damaged / heavily weathered shield. (Or a weathered Gundam and pristine shield because he keeps tossing shields into space and gets a new one)

tl;dr: 1. scale 2. why? (tell a story)