What's everyone's recommendations for undercoating?

I'm a northeastern dweller, so this conversation is naturally coming from a rust-prevention standpoint. Not that the D cars will see salty winter roads, but will see plenty of rain I'm sure.

As the weather warms, I plan on diving back into my S13 project. Last fall I completed the cutting and welding of much new metal to perform a rust delete. As of now the new metal is in, seam sealed, and primed. I'd like to at least coat my wheel wells and forward sections of frame rail with a permanent solution.

I'd like to hold an open discussion regarding products and practices. Body and paint are another language to me, I'm all nuts and bolts. Looking to nail down a reasonable but not counterproductive approach. Things I'd like to address are:

Luckily since it's underbody I'm not worried about prep perfection as things like runs, orange peel, etc. aren't overly a concern in comparison to a paint job. I am just overly paranoid about applying a coating that "locks rust in" or allows the chassis to rust from the inside out, as I've heard so much about.