I would also go with the matt clear coat. The gloss tends to emphasize textures. The thing is that even the absolute best brush painting will not compare to the quality of a spray paint, even from a can. Not only that, even with the time spent masking it will also end up being a lot less work too. I can still remember the "Ah ha" moment when I switched over to spray for doing model car kits. The difference was astounding immediately. I still do much of my weathering by hand though, as well as detail painting and dry brushing. Just remember when using a spray can to make quick (the quicker the thinner the paint) complete passes over the object and don't hold the can on the object when spraying or it will be too thick and drip, as well as fill in all the detail. Main rule of thumb, the thinner the paint, the less it looks like paint, and will like it is molded in that material.
Using a flat black primer as your main paint is also nice, because it takes dry brushing and buffable metal paints nice. Many pros use that with just some rub and buff to make quick metal looking non-hero weapons. Some of them have it down where they can be shown up close and look very real too.
Here is an example of the idea from Rick Ross...
http://pflagerkatsumata.com/PKD-Enforcer_2.jpg
And as PHArchivist said, sanding. Make sure your base model looks smooth, and clean before hand. Do one thin primer coat, sand, and then repeat as needed. Hand brushing will still work geat though if you are trying to add rust an corrosion on it.
KramStar has some tutorials on that I think on his site, here is his RPF thread...
http://www.therpf.com/f9/blade-runner-pflager-katsumata-pkd-pic-heavy-61915/
Some pictures of extreme weathering by Rick Ross on Propsummit...
Propsummit.com a Blade Runner Prop Community Forum BladeRunnerProps.comView topic - Rick's PFLAGER KATSUMATA SERIES D Magnum (showing off)
Hope this gives you a lot of cool ideas to experiment with.
Andy
p.s. Do you have any pictures of it before you started painting? I would also like to see the otherside too please.