Lower your oven temp to 325 to 350 and still bake for 30 min then open door and let cool slowly. This should help butthe problem probably comes from powder with moisture in it or the powder wasn't ground fine enough to begin with. You may have oils on the surface even after cleaning,simple solution is hang the part in the oven while it preheats then once up to temperature remove the part,allow to cool then apply powder.
Let us know how it turns out.
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