Addressing some rust starting in my truck bed

[Jeepwm69]

Removed my bed mat a few weeks ago and found this. Cleaned the bed thoroughly and figured I'd hit it with a wire brush on my angle grinder, phosphoric acid, and then clean and paint with primer.

Anyone have any advice on a different/better way to stop this in its tracks?

I've let Jeep floorboards get out of hand and don't want to do the same with the bed on this truck.

Obviously, the seam is what I'm most concerned about stopping.