What you see on the outside is happening 10x on the inside. I cut my frame in half and the inside of the frame was filled with rusted metal flakes. the flakes fill up the bottom of the frame tube, and hold in moisture, which accelerates the rusting from the inside out.
Fluid film is a popular one to slow down rust, so is diesel. But try to direct some inside the frame, thats where the real problem is.
Keep an eye around any brackets, pushing the area with a screwdriver to locate any serious problems.
Then buy a welder to get you an extra year.
Then buy a new frame