The back arches on my '99 Focus are starting to get a bit bubbly and some bits of paint are flaking off, what would be the best thing to treat them with before they would be painted?

 

Take the rear wheel arch liners off and have a look in there first, if the rust has got to the outer wings, I bet the inner arches are a lot worse!

 

agree with above rust is a desease normally best way of resolving it is to cut it out and replace with metal

 

Cut it out start again anything else just buys a little more time

 

This^ sad to say its the only real solution.

 

This will maybe give you a better idea

 

I've had some success with rust treatments.
Bilt Hamber do the best stuff without a shadow of a doubt.

I done some work on a MK1 Focus about 2 years ago. Arches were far worse than yours. I cleaned the car and degreased all the inner arch and the car itself. I used a few wire brushes and flap discs on a grinder to get back to decent metal.
Treated all the bare metal with Bilt Hamber Deox Gel to ensure there was no rust left behind.
Sprayed the entire inside arch and the now ground back edges with epoxy primer. Put fiberglass behind all the holes in the arch and once set I skimmed the holes with filler. Prepped and painted the outer arch. Used Bilt Hamber Dynax UB on the inside arch.

Seen the car about 2 months ago and still looks absolutely fine.

 

I break 170s so I see a lot of mk1s and yours is worst focus I have seen for rust so far on the arches, that looks like it needs checking on inners as richie said at start but that looks bad

 

Ok will get a better look then when I get my Vectra back on the road as I'm currently using this for work.

 

Looking at the way that's bubbling are you sure it's not had repairs before?

 

Most focus.es are going like this now. There getting to be an old cat now. I did the wifes focus wheel arches over a year ago and they are starting to go again.

Ive seen some W reg focus.es and they look mint compared to the wifes focus.