I tried this when I repaired my floor pan and I tell you what, it didn't seem that weldable to me

I ended up wire brushing the paint off everywhere I had to weld

 

If you're looking for a use for the PB, spray it everywhere you don't want rust including brake lines, door drains, rocker panels, etc. It might help if you can mix it with a heavier oil that will last longer. PB by itself is pretty thin and probably won't last long.

I've been using a product called Fluid Film for years for similar purposes. I spray it inside doors, fenders, undercarriage, etc.

 

I'm not really looking to waste it. I just have plenty available and needed to protect the large surface of bare metal.

The PB worked alright, not great. Doesn't matter now, though, as the welding is nearly complete.