mine has never been kept in a garage or under cover since 1991 and only has a few rust bubbles on the roof and a small patch on drivers front and rear doors, so it can't be that bad for them to be left outside, I have done a couple of small repairs to the usual places ie: foot well, prop tunnel and a small patch on the rear bumper mounting bracket and thats it, the arches are rot free and the chassis rails and the rest of the underside is spotless,

Not bad for a 25 year old ford.