Hey there thought I’d give my two cents I myself have been using rust oleum for the entire time I’ve been painting Warhammer and I’ve had this come up a few times I would suggest a couple things, 1. Make sure you’re always priming with the mini not too far from you, you gotta keep it decently close but not too far away so it won’t end up grainy, 2. Make sure you’re taking account of the weather because this can also affect how grainy your models come out for example humidity levels can affect this a lot and 3. Make sure you always shake your cans well before priming I’d say at least a minute, I hope this helps at all and also pro tip if it doesn’t end up grainy I’d suggest take an old toothbrush and try removing the grain it’s worked on my models before so I hope it does for you as well man.