how bad is this rust ?
 

how bad is it - in real terms and
whats the best way to go about fixing it ?
grind it off ? seal it with fibreglass and resin ?
excuse to put widebody front fenders on ?
flog it as the cancer is bad and gonna just get worse ?

this rust is on the lower rocker just behind the front wheels on both side
the rear rocker is fine

That's nothing. Fix it right, and you'll never have to worry about it. Cover it up, and it'll eat through from undeneath.

Grind/cut it off. You need to get rid of all the rust, every little bit. Use acid to get into the pores of the metal for prep. DO NOT skip proper prep. If you do it half-assed, you might as well let it go or do nothing.

Don't cover it, you must remove all the rust for a proper repair. POR15 works well once you get the majority removed.

That rust does not look to bad. It's contained in the front fenders, so you might have to replace them, but you won't know until you take them off to see what's going on behind them.

The reason you get this rust, is because the drains for the windshield cowl sit above the front fenders, and any dirt and debris gets caught behind your fenders, and starts to rot/rust.
Now, you might find that there is rust on your body too. meaning that behind the fender, the unibody of the car is starting to rust. This happened to me, but that was after driving my Miata through 6 years of New England winters. Good luck.

-Jim

ok so it seems fixable - thanks for looking and the advice
Now I gotta go loosen the fenders where they join the main unibody and see what its like in there
then grind away all the rust and replace with sheet if appropriate

the car was a coastal car but now its in a dry climate

Surface rust, once wire brushed to remove the loose stuff, can be coated with POR15. It's a rust conversion primer, and is very highly regarded in the locost community. It will need to be painted over on exposed surfaces, but inside the fenders it is fine by itself.

Make sure to clean out all the drains, to avoid gunk building up in there and holding mositure.

As long as the fender is off . . . Frog Arms?