How do i fix a rusty wheel arch on my mk3 fiesta?

Depending how bad. I did my Sierra which lasted until a coach hit it and reshaped the back [about 2yrs].
First check the rusting is not an MOT failure ie it could be if it is through to the inside of the vehicle. But if not this is what I did on my Seirra
Clean the arch of any loose rust and dirt [very important].

Treat rusted areas with 'Rustkill'. Poprivet or use self tap screws to fix body repair gauze to sound metal work and buil up with fibreglass and paste mix, this give the fibreglass and paste a good foundation and adde strength.
If any riveting is showing on the outside of the arch ie: it should only be the rivet/screw heads. then indent around them so they sit below the surrounding bodywork [do this with a small ballpein hammer. then fill in the indents after scoring the metal,with body repair putty. smooth flat this will hide the rivet heads. Paint the inner section of the repaired arch with underseal. And any finish to the outside bodywork should be done with red lead bodywork paint, about 3 coats, then 3 undercoats and two/three finish coats, each coat smooth with wet and dry paper. T.cut the whole car a little area at a time [don't ever let it dry or you'll be there till next christmas getting it off]. and it will as near as possible blend in with the new paint. Unfortunately rust repairs have to be checked every so often as they do 'open up' because of viberation.
If you want send clear description of where and how bad the rust is to I will then send a drawing back of how I did mine repair. The other thing to check is Wheel arch repair panels for wheel arches that have rusted through on the outer section, if they still do them most car/bodywork shop will order them. show them where and how bad the rust is first. Remember even the best has to sand down the fibreglass filler so don't expect to fill it perfect first time. Use a medium metal sanding disc to take down the worst then fine,followed by wet'n dry paper.

Get it from car/bodyworkshops. Wet and dry paper two grades coarse and fine. or the wet and dry blocks.
Poprivet gun some car shops hire these.
'Rustkill' or similar from most car/bodywork shops
Self tap screws with small heads car shops.
And obviously either a car shop or body shop for a fibreglass kit.Same with the arch repair panels.

Luke Smith
January 2005