i do an assload of powdercoating for my job. the biggest piece of advice i can give is you really cant get too much powder on there. i have never gotten a run or sag with a room temperature part. if you pre-heat then you have to be careful.
also, bake it longer than 15 min after flow out. closer to 30 min is good depending on the part. thicker cast iron pieces need more time. i did some brake calipers recently that were in the over for an hour total.
one last thing, that HF gun works but you get what you pay for. eastwood has a nicer gun for just over $100. that one you have will do, i have used it many times, but you will notice the difference just spending $30 more on the gun. eastwood is a great place to get powders as well.