I did powder coating on the side one summer and started out with one of those, upgraded to a much fancier deal with adjustable kv and amperage. They work ok for single coat/single stage and relatively simple parts, once you get dual stage, or parts with a lot of corners they really start to struggle
warming your parts before spraying will help immensely, not 250f, like 100-200f. It will help get rid of any moisture, and the powder will stick much better. Dual stage or multiple coats with that gun you are going to have to "hot flock", basicly pull it out of the oven, and when it cools to 250-300f, spray your next coat/stage/layer, you may need to do that even with tight corners.
Good ground as mentioned, clean, any spec of grease from your hands or otherwise will show up as a fisheye. You cannot coat over a fisheye, it will show up in successive layers. Prismatic powders is a good spot for powder, they have a huge inventory, decently priced, you may want to order a few colors to offset the shipping (although not very high).
Once you get the hang of it, check out some of the multi stage powders, some really cool effects/colors are available.