9 years is a long being scared spraying it on the frame and tire walls without even trying it on the paint.Your windsheilds is the only glass you have to worry about,if you respray it with soap the splash from appling the HF will do no harm.You do not want to apply HF to the top of the truck as for it to run down on the windsheild with may cause harm.
To save chemicals and the way you want to apply(downstreamer) you need a metering valve to ajust your mix.Even on your sur-flows could you not pull water though the intake and use a Y fitting in the intake to pull chemicals too.With a metering valve on the chemical line I think you could ajust to each truck being washed,more or less.
When In Doubt Think It Out
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