Geg,
The one issue I have with foaming the bumper is the possibility to trap water,
Pre-treatment for rust is the best preventitive option.

 

Por15 is good if you pay all surfaces.

 

also thought about it ... so I stopped ...
water is a tricky substance, it will always find its way even through a small hole, but the foam may not allow it to come out or evaporate

I also had a desire to drill a few holes in the bottom of the reinforcement so that the water flows freely outward. But on reflection, I realized that it was pointless. The road - not plumbing, there is no clean water. The bumper will become clogged with dirt, the dirt will absorb the water like a sponge and prevent the water from flowing out quickly. Thus, the holes will not be a salvation.

Only timely prevention / treatment remains. It's damn inconvenient, for this you need to dismantle the bumper. The only good news is that this is a very thick iron - it will rot for a long time, thanks to the American legislator.