What machine are you using ?
Does your wire always feed smoothly ? If not you can go up a size on the tip. Some brands of flux core wire aren't very clean and uniform, causing inconsistent feed and sometimes gets stuck in the tip. I use an .045 tip with .035 flux core and it works perfectly.
You need to keep a tight arc and move at a constant speed... and at the very least, brush the loose rust off but it's best to clean it with a grinder.