I have had many project cars. One thing I found interesting ( in a bad way!) is how I could have a rust free car, da down to bare metal in that pesky lower rear quarter, and BOOM! I see all kind of rust "pinholes". YEP... Rusting from the inside. Only real cure is cut all that OUT all tht bad metal. ALL of it.
I have a car buddy, that restored his 67 GTX hemi car, leaves it in an enclosed pole barn, limestone gravel floor. He does not look at it for 2-3 years. The rechromed bumpers had rusted to heck, rust trying to appear her and there on the body, mice pissed in the interior, carbs were bad......HUMIDITY!!!! and that lime gravel draws moisture in that building, and the prigging car over!!!!