If you use cold rolled mild steel with no coating, make sure you prep and seal all the surfaces. Where you're mating two layers together with rivets, coat with a cold galv spray or something that reduces corrosion.
I don't think I've ever seen 16-18-20 gauge hot roll sheet steel. I highly recommend electro galvanized sheet steel if you can acquire it - you'll have the least worries about needing to apply corrosion prevention. Most bus/truck/fleet sheet metal applications where rivets are used employ electrogalv sheeting.
Originally Posted by coach21
I have raised my roof and am now in the process of buying sheets of steel to skin the bus. Should I use hot rolled or cold rolled steel? The price difference is only $2 so that's not an issue. I just want to get the right one.