The reason craft foam has to be sealed is because it absorbs. If you paint it with acrylic before sealing, it will absorb the paint. It will look flat and the color won't look right. It will be especially difficult to get a smooth, metallic finish.
I have personally used plasti-dip on craft foam, on a time crunch. We cut foam armor, sealed it with 2-3 coats of plasti-dip and painted it, all in a day. The foam stays surprisingly flexible. Make sure you heat shape everything and reinforce it so that any flexing doesn't crack your paint finish. Here's how it looked, in the end: Thor armor
In the end, craft foam isn't a particularly durable material. If you want a costume to last, look into other types of foam.