first what mk of fiesta is it mate, the only real way to do it is to dismantle all the suspension clean it all up either paint it or whateva and wipe some thinners or petrol over the rubberrised underseal ford put on ,
to get all the shite off ,

then as you say primer it all paint it the colour of your car and a good couple of coats of laquer,
while you have got it all apart you could poly bush the whole car , either do the rear end first then work your way to the front, have a look at twins thread on here his underbody resto is amazing, hope this helps. mark.


(ps for fock sake dont just paint hammerite ontop cus it will look shite)