I need to do mine too...did my dad's gmc the other day, took the bed off, cleaned it all, painted it all and the wheel archs of the bed, it looks great. The wheelarchs look terrible when they're all chipped and ****ty, especially since GM color matchs the wheel arch and bed struts (little supports that go to cross member from box side)...

 

yes and no, rust is iron oxidizing. So if you take out the variable of oxygen, then it can't rust further.

Thats like having a small camp fire, throwing a big thick blanket over it. It will eliminate the spread of it, but the fire was still there and the evidence was still there.