Ken,
You have a compounding problem... the issue is not your For() loop, it just looks weird because you didn't use brackets.. but it works fine for one instruction.
The issue is that you never clear voltraw, before you start adding up your 20 reads. The first time through the loop it's fine, but successive times it is over flowing your INT variable, and then dividing by 20 still.
Just put voltraw = 0; before your For() loop.