Applying Rust Killer to under carriage /frame any reason not to ?

Neutralize refers to "returning" to a pH level of 7 or somewhere thereabouts.
pH scale runs from 0-14. The lower the number the more acidic. To counter act the "acid", usually its wiped/rinsed with something "basic". Basic is the higher end of pH scale(lye) up to 14.
Hope that helps?
What do folks mean by "neutralize it." Do you rinse with a base solution?

I've used Ospho to prep a lot small parts. All I've ever done to prep for paint is rinse the part with water afterwards, dry it, wipe it down with acetone, and on to paint. That process has worked great for me.