Finrot is a bacterial infection and can be treated the same way that Flex is treated since it is one of the bacteria that is associated with finrot.
You can treat a couple of different ways. One way would be to do a malachite green dip. The other is to QT
the infected fish and treat with an erythromycin based antibiotic.
Malachite Green Dip
Malachite Green is the main ingredient in most Ich remedies (such as Quick Cure). To perform the dip, fill a container with 1 liter of dechlored water matched to the temp and pH of the main tank. Add 1 drop of a malachite green based Ich remedy. Carefully net the infected fish out of the tank, while still in the net, dip the fish into the solution. Leave the fish in the solution for exactly 20 seconds-no more, no less. After the 20 seconds have expired place the fish back into the main tank. Repeat this treatment every other day for two weeks or for a few days after any sign of infection is gone.
The reason the fish is dipped for no more than 20 seconds is because the solution is highly concentrated. Most Ich meds are dosed 1 drop per gallon. While this is strong enough to kill the Ich parasite it isn't strong enough to kill flex (which is essentially what finrot is). Adding 1 drop of the med to 1 liter of water concentrates the solution making it effective against flex/finrot. However, due to this concentration, leaving a fish in for too long can kill the fish.
The other option would be to place the fish in a QT
or HT and dose with an erythromycin based antibiotic. This is an effective drug and is what Maracyn is made up of. The Maracyn is the main trreatment. Maracyn 2 will kill off any secondary bacterial infections that may be present. Dosing is as directed on package. These drugs are photosensitive so the tank lights should be turned off. They will also cloud the water and an airstone should be added to the tank.
Treat for 5 days following the package instructions for dosing. After the fifth day, perform two 50% water changes 12 hours apart and repeat the treatment for another 5 days.
Even if it looks like the infection has disappeared it's always good to treat for a few days longer just to be sure.