The beauty of using the compressor and spray gun is that you can get an extremely fine mist which gives great coverage and is very economical in terms of how much product is needed for the job.
My concern would be how thoroughly the job was done by a 3rd party, ie bike valeted / ACF50 pre winter (but some component missed), wash the bike off post winter and find the missed bit totally corroded. These things are ~£50 from screwfix which seems alot cheaper than buying a spray head and separate compressor ~ but any good....? https://www.screwfix.com/p/erbauer-erb561srg-700w-electric-hvlp-spray-gun-220-240v/11628