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I've heard about this before when using pastel chalks. I trialled it for awhile but ended up using pigment powder and mixing it with some thinner and a bit glue. That way the dirt stick. I ended up doing that for my M1A2. 
That's fine, then. If you have any method that works for you, then use that instead. There are a bunch of ways to do any of these techniques, so pick one that you like best.
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Im thinking i will probably gloss the whole sherman anyway as if it isnt obvious, the decals werent sealed in as i was under the impression back then that the decals would stick there forever.
While you can gloss over the decals, it appears that you are getting what is known as silvering under them now. A gloss coat will _not_ clear this up. Depending upon how much work you are wiling to do, you could remove the decals (use tape to pull them off and/or gentle scraping with an Exacto knife) gloss coat everything and then apply new decals. They will not silver after that.
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can you guys suggest what i need to use to thoroughly clean any reminents on the airbrush?
I suppose I have to ask what you are using to clean your airbrush now and why you think a clear gloss will cause any special problems? 'Cause it won't.
I use both a single action Paasche H and a double action Paashe VL. For both, my normal cleaning routine is to run some of the thinner used for the particular paint through the brush (for Future, that would be water), and then to break down the brush and clean all the bits in laquer thinner. I don't care what paint you are using, dipping the metal bits in laquer thinner will clean them up perfectly. No problems. In my humble opinion, you just have to break down the brush each time you finish painting. If there are any rubber O-rings, remove them before dipping that part in laquer thinner.
Occasionally I will just run some thinner through if I am switching between similar colours or going from a lighter colour to a darker colour during the same session. But at the end of the session, no matter what I have been spraying, I break down the brush & clean everything in laquer thinner. On the H it's really easy. On the VL, less so and that's probably why I use it less. Still, I clean anything the paint has come in contact with and I never have a problem with one colour showing up in a different paint job or of any parts sticking or jamming.
Paul
