I mainly PC aluminum stuff that I make. I’ve been having trouble getting the finished product to be smooth and free of impurities.

That is an example of what I get all the time. I’ve read that aluminum requires more heat/curing time because of how easily it dissipates heat. The powder calls for 15 minutes at 400.
I typically…
Clean the piece with Prep-all to remove any oils/etc
Preheat the oven to 475 or so.
Insert piece once oven is up to temp and let it sit for ~10 minutes, then cure at 400 for 20 minutes.
What am I doing wrong?
I always get those little piece of trash in the finish, some items more than others (it seems to be sheer luck) and the finish is never 100% smooth. the picture is an accurate representation of the finish I always get, it’s somewhat close to smooth but not quite.
Do I need more heat, more time, preheat?
any help is appreciated.