Spray the coating, works better than the snake oil. If doing that you first use a spray gun ( known in the spray paint trade as a paraffin gun) to put a mix of 80% boiled linseed oil and 20% paraffin/kerosine/jet fuel into every box section of the vehicle, removing all the plastic caps if present. That will provide a very good corrosion barrier, and stop existing corrosion inside from eating through. You do need to have the underside clean, preferably using high pressure washing with detergent and a high pressure rinse. It will drip for a long time afterwards, you will need to have a big cardboard pad where you park to absorb the dripping as the paraffin evaporates.
Messy, but an incredibly effective job for the cost. I did this to my last van, and after 10 years the little rust spot it had when I got it was still the same little covered spot, and I live in the corrosion capital of the planet. Plus it used to get driven into the dam every weekend with a boat, and spent a lot of time on the beach.