I'd like to start this by stating that I am an amateur, but hey, we all start somewhere.
After I had repainted my armour and gotten rid of the 'chocolate chip' weathering I had done originally, this is how I weathered everything:
I took my forearm for an example
And I took a scraper and a combo of rough and fine sandpaper (you may not want to use rough, that's fine) I used rough first, then finished with fine.
Remember, less is more.
Then I gave the whole surface of the armour a light sand with a fine grit sand paper
After that, I took Black Acrylic paint, and a VERY wet paintbrush and brushed over it, before quickly wiping it off with a towel, I left the paint sit longer on parts I wanted to be dirtier, mainly the coverstrips and the raised section of the forearm.
I feel I need to add here: DON'T BE AFRAID to completely coat the armour in the paint, there were points where I just dry brushed it onto parts to give that section a really dirty look. If it was too much I was able to wipe it off quickly, and if I still wasn't happy I found this product worked wonders:
It can completely clean the armour if you want to start at the base colour again, just add water
Here is the finished product:
Here is a comparison of weathered and unweathered forearms
I didn't use any type of brown washes as I don't think they are needed, and they don't appear to be present in any of the reference photos I have seen. Some people have used them to give the armour a more 'dynamic look', in the end, its down to personal opinion.