I also like Oxpho Blue better than Permanent Blue. I found Oxpho a little more durable.
Since you are doing large items, you are going to find it a challenge to get the finish even and streak free. The odds are against you in that regard.
One way is multiple coats. Get the first one, degrease the metal, then use degreased steel wool to polish the coat, then reapply and you may find yourself doing that 10 more times before you have something "acceptable" to you.
A more durable finish is molybdenum. You can get it from John Norrels, they market theirs as moly resin. You will need an oven big enuff to fit your barreled receiver if you want to go that route
