I'm also playing catch up.....
Great to read you have been accepted into Citadel, you must be well pleased....I wish you good fortune in your chosen career.
I've just been refreshing myself with your build, it's coming along really well.
Colin's advise is sound, so no point in me confusing the issue with another perhaps slightly different method. You will find most folks have their own pet way of weathering, so why not have a play on an old model, before committing to the 'Real thing'?
I don't use bi carb. so can't help there, sorry! I just pick out the rusty bits with a bit of brownish, thinned enamel, then gently ease it downward with a dry brush....Bit basic, but it works!
Good luck with the painting, whichever method you adopt......
Ron