I am in the process of painting my jeep with Raptor liner. I am spraying it on my frame and under the tub and I was thinking of using POR-15 as a primer. My frame is full of rust, but the underside of the tub only has mild rust spots. My concern however is that I also have some paint and undercoating that won't come off. So my question is this; Will the Raptor liner (which is urethane based) stick to the POR-15? Will the paint and the undercoating cause any issues. Is it worth using the POR-15, or would it be better if I use a self etching primer?