Get the right tool made for the job, a 6"DA (dual-action) sander, airpowered. A selection of 80 thru 320 grit pads for it. And get an air powered board sander - 80 grit is what I use.
You are finishing metal getting ready for paint, take your job seriously.
I'd also get a 3"DA, they are one of those items that after you purchase one you wonder why it took you so long.
These tools are basic to get your project into primer, you'll need a few others for finishing.
You can find these tools, used in very good condition on ebay for a bargain. You can buy used with some confidance since they are durable tools and seldom wear out, stick to name brands.
