That's a shame, your's is such a smashing example, but I'm afraid age catches up with them all eventually.. I've got a little bit developing on the nearside rear arch too, the site of a previous battle scar. I'll have to attend to it when the weather's a bit better and I have a bit more time.
That stuff does work wonders though, clean off as much as possible before applying multiple coats. A quick sand and repeat until no sign of rust... then a smear of Fibrofil to smooth out and seal the surfaces before final sanding, paint or body shutz. It's just phosphoric acid in a gel, brick cleaner works too.
Should put it on for a few more years.