I don't see the point in diluting phosphoric acid. I pour it straight into aluminum turkey pans and soak the parts until everything is nice and foamy, ten or twenty minutes. Then I pour it back into jugs until my next rusty part is ready. I'm as cheap as the next guy, but I find diluting just confuses me, since it doesn't seem to loose it's properties, and I need to know when it's full strength (allways).
If you need a five gallon tub of this stuff, you should really consider treating one corner of the part at a time. You can spray too, but it seems to dry before it has time to react.
Whatever is unwanted, give to somebody with one of those Arrow metal sheds or a lot of iron railings.