You could put a ignition resitor in line with the fuse for the sprayer, you'll have to get with your trusty auto parts guy/or gal to determine which one you'll need.
Tell them your droping DC volts with it so the make and model dosn't matter, you'll need to look in the 68 to 75 years, tell them you need one for a 70s mopar, they usually drop volts by 6 volts.
Also be sure you don't have some fudge room on the volts, volts don't burn stuff up, amps do
so check that out as well