First of all: what infection is it?
I've read this: https://www.wearecellix.com/post/microbes-that-ruin-a-perfect-pint
and yesterday I opened an old bottle with an infected homebrew. A lot of diacetyl and off smell, not vinegar, but like vomit. So I think it's Pediococcus or Clostridium. I don't know if these make spores.
If they make spores, that's the cause: I used iodophor which didn't kill the spores. And where did they hide? In the ball valves. That's the only place I couldn't get to. (I used citric acid in iodophor in cold water and left it for an hour because I read in wikipedia that the iodophor, in order for the iodine to be active, needs a pH between 2 and 5).
So I know the cause and I know the site of infection. Somebody used a heating element to boild the water inside the steel fermenter. I think that's a good idea. I might try it. I don't know if the silicone gaskets will hold, though.
The reason I think percarbonate in the full fermenter overnight would be a good idea is this: if I don't raise the concentration of the disinfectant solution I can prolong the time the disinfectant stays in contact with the surface. I have been using it for a contact time of one hour, maybe 12 hours will do the trick.
And why I think the ball valves are the cause is this: when the ball valve is closed, there is no contact between the wort and the outside, it is hermetically sealed. But when I take a gravity reading, I open the ball valve, beer flows through the open valve and when I close it, wort gets trapped inside that valve which means infection. And when I open it again, that trapped infected wort will ruin my whole 5 gallons of beer.
What I am going to do from now on is this: I will brew my next batch of beer and all the beers subsequently in the same manner.
I will fill my fermenter with sanitiser and leave it overnight fully closed.
I will weigh and mill my grains one night before I will brew.
On brew day, I will take only one gravity reading, the preboil gravity and that's it.
Then I will ferment my beer the same way I use to, but without ever taking a gravity reading. The only gravity reading I will take will be on bottling day or not at all. I ferment at very high temperatures, so I never had a batch that didn't fully attenuate.
The airlock has been replaced with a blowoff tube in a jar of vodka. This lets me see how much suckback I have.
And I think I'm good.