Rust in the soil

After the identification of the rust on my geraniums, I have realised it is what is making all the leaves fall off the buddleia.   I shall cut it right down today, but it also advises treating the soil.    There are containers that I can buy and dilute, but all the reviews say the tops are terrible and they have to cut them off etc.
Then there are ready mixed spray bottles (expensive).    Bordeaux mix and similar are out there, but I don't want my rash to kick off.   I read about using soluable aspirin.  8 tablets dissolved in 1 quart of water.   Have you ever done this, or does something come to mind which I have overlooked.   Thank you.