.
...I personally think you're better off on the internal treatment with some sort of oil based corrosion inhibitor product. Not certain how you plan to neutralize the phosphoric, and even after you do, the spots treated are still open to oxidation.
Something like framesaver or this stuff.
Rust-Oleum 224284 Stops Rust Rust Inhibitor 10.25-Ounce Spray
If you insist on clear coating it, my preference is a clear enamel, not a lacquer. But I do use lacquer paint for touch ups on some frames because it comes in small spray cans in so many convenient colors at the modeling store nearby.