Mig weld a M98 body? Ain't gonna happen.
The Tippmann Model 98 body is a heavily alloyed aluminum, with large percentages of (probably) tin, zinc and magnesium.
This sort of alloy is usually called "pot metal"- it's easily cast, has decent strength, takes detail from the molds well (remember the term "die cast" for things like Hot Wheels cars- the molds are steel dies and have a lot more detail than, say, sand-casting) and it's dirt-cheap.
The "cheap" part comes from the fact it doesn't have to be a high-quality alloy. If one body has 10% less tin than the next, or the end of the run has 5% more zinc than the start of the pour, it's no biggie. What strength it has will be barely affected, and there'll be little cosmetic effect. They literally make it out of scrap- cheap, ungraded mishmash remelted aluminum scrap, casting dross from higher-grade smelting processes, etc.
However, due to the odd and iffy alloy, it can't be annoed or welded. If you try and weld it, the various alloy fractions will try and melt at vastly different temperatures. The tin will melt at about 500 degrees, the Zinc at barely 800. The aluminum won't melt 'til 1,200 plus.
So what happens is the lowest-point material melts first, and the not-yet-melted stuff crumbles away and fractionalizes (tries to seperate.)
So, short answer to the question, no, it ain't gonna happen. Use glue or a mechanical attachment. (Screws, etc.)
Oh, and the anno is the same thing- the non-aluminum parts tend to be dissolved in the anodizing electrolyte bath, so the aluminum tends to crumble away. Zinc and magnesium both will be corroded away by the chemical/electrical action.
That's why they're painted from the factory.