sanding back surface rust

It was time for my trusty beater's spring clean and i noticed the surface rust has got a lot worse from a year ago. Its still been wiped clean after wet rides and i clean/maintain components as and when they need it, so this isnt a leave outside and let it rust into the ground problem.

So wondering whether its time to sand back most of the rust and spray it down with a clear coat to get at least another year out of it while i potentially look for something else. My question is which parts warrant sanding? Obviously the larger patches of pitted bare metal, but where the paint is bubbling or looks veiny, do i just leave that or sand back there too. The frame is so ratty, as youll see in the close ups, im worried ill end up sanding away 95% of the paint.

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i would just scrub the rust off, using bilt hamber deox-gel (or something similar) and a brass brush, treat exposed steel with rust ol (rust converter) then clear coat / nail varnish over the top of that.

you have to degrease and clean it pretty thoroughly beforehand for all that stuff to work as well as it should

Wire brush on a drill, about an hour of drill action, attack anything thats bubbled up.

Did the worst bits on my raleigh, the fork looked really bad, worse than your chainstay, took the whole fork back to bare metal and it came up really well, just grey primed it after that.
Bubbling paint on the chainstays was just the tip of trouble, the rust had spread under the paint but not visable until i started scraping.
I wasn't happy with a rusty frame that looked worse than it was (didn't know that until i wire brushed it)
I reckon the paint job is dead, a repaint would add years of life depending on whats hiding underneath. Attack it now, clearcoat or prime the rust free bits and decide wether to get a new frame or take it back to bare metal and respray it with rattle cans at home.

The rust looks only surface based, and no flaking so that's good. The rust will only be microns of steel eaten away so no problems structurally. But wire brush and a rust neutraliser is required to prevent bubbling after the repaint.
Internal treatment is recommended at this stage, frame saver is the choice product but dinitrol automotive products are also great and cheaper.

ive sanded back a few patches with 180 grit sandpaper just to see whats going on. metal looks pitted and the veins under the paint are indeed rusty. tried a small section under what looked like smooth unblemished paint and that was also mottled. so looks like i would have to sand down the entire frame by the looks of it. would painting over that mottled/pitted bare metal just be covering the problem? is that rust thats just going to keep eating deeper into the metal?

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yup it will come back pretty fast unless its treated with chemicals, imagine a mountain range, thats what the metal looks like under a microscope. It needs media blasting or dipping in acid. Otherwise as people mention cures then theres also kurust and Jenolite but be careful using these types of product to avoid splashes on skin or in eyes.