okay.....quit using that sander.
Try to find a DA sander (dual action) for cheap....
A DA sander evenly sands the surface better by constantly moving the sand pad around. The sander your using, which im assuming has the side to side motion will only sand on one spot of the sand paper unless your on a flat surface. This can cause imperfections in the body and in most cases, flat spots
What your doing is working, however you will get better results and a lot less work with a DA sander. I prefer the palm ones but the ones like the picture above are usually cheaper. If your near a harbor freight tools they should have one for pretty cheap along with the sand paper.