Nice, my granddaughter and I built almost the exact same bumper a few year ago. We had a great time talking about the design, as you can see we decided to ditch the rainbow unicorns in the final version.
Might want to look into an injection gun, I used ValuGard Rustproofing on mine but it would spray your Penetrol just fine and let you “penetrate deeper in to the voids” if that’s important to you.
This is the gun I have.