A couple months ago I did a big production run of stoves.  It was just me and one helper and we had a bit of a deadline to get it done. 

I found on the CNC plasma forum a good way to prep steel for painting is to use muriatic acid diluted with 4 parts water in a big plastic tub. 

Works wonderful, just soak, wash, a little scotchbrite pad and shiny bare metal.  If you are going to powder coat, this method really gets mill scale off the parts.

We did over 50 stoves in 2 days using this method and we did not need to use the expensive flapper sanding discs once.

Just be sure to use your safety gear, apron, gloves, face shield/goggles with this one and do it outdoors.

BTW powder coat is the most enviro friendly method of painting as it emits no VOC.  It is also faster and more durable than paint.  one hour from start to ready to use item.  Amazon has a kit for 35 dollars for the gun.  We use a old toaster oven that we cleaned up and hit with some high temp paint for the curing.  it works best on 400 watt setting.