I just used some hose and a (very clean) dish soap bottle filled with flush solvent to flush out my A/C system. You can put solvent in the hose and blow it into heat exchangers with compressed air that way. No need for a flush gun, or nylog, just coat O-rings in mineral oil... that is what the local A/C shop in business for 50 years told me.
I suppose you could remove the condenser and powerwash it/ paint it with evaporator conductive paint. If you strip out the top brass nuts on the expansion valve, you can clean the evaporator coils when you remove the evaporator box for a new one
Pretty sure CarQuest or NAPA has good flush solvents in quarts, not in a spray can. 2 quarts were enough to flush everything out well for me, got those for $9/quart from my local A/C shop.