Is it okay to use POR 15 on a primed surface and does it come in mat as well as gloss? I have used the gloss and it has been great but the finish I would like is low sheen or mat.
You can use POR-15 over a primed surface but it's not the beast way. POR-15 wants to ideally be direct to metal.
If you want a semi gloss then you could kill two birds with one stone here. The Chassis coat is a semi gloss and a top coat which will go over a primed surface better that the rust paint which is only gloss.