whats the best way to prep my defender for spraying is it best to take it back to aliminum ?

Rub it down, go down to ali where there is bad corrosion and deal with it then prime it. Use acid etch primer if you want to get a really good primer coat or just use ordinary primer for a job that will be fine to be honest. Depends how much you want to spend and how long you want to spend on it and what sort of quality you want.

Use acid etch on any areas you do need to take back to metal, steel or ali. I got a big can of it from Halfords for my wing repair (rubbed it down to ali, pulled a dent, filled it) - or you can buy tins of it - if you're using a spray gun.

how mcuh it cost

acid etch is about 28 quid for the etch primer and activator 1 ltr. if you want a good finnish i would say use a DA and get down to the factory base coat. as advised discomania any corrosion should be taken right down to the bare aluminium and if it is pitted use a spot sand blaster to get the area as clean as you possably can and etch prime straight away.

most acid etch primers have a shelf life once they have been activated so make sure you have all the 'bad' bits ready for etching in one go, once the bad bits have been given a coat of love and has dried etch the whole of the vehicle then use a standard primer over that. you wont have any problems with paint comming off if you go to the trouble to get the base coats right. its time consuming and a pain in the arse but the finnished results are very much worth it