18 gauge? that's thick! i personally haven't come across anything on my 69 that thick (not including items like frame rails, etc). 18 completely sucks to cut and form (but is cake to weld). are you measuring metal with paint on it?
Thick? 16 ga is thick! 20ga is thin and 22 ga is paper thin
Yes, mine was measured WITHOUT paint, sealer, etc. Floor pans, firewall, inner fender aprons, "B" pillar, roof panel..all were 18 ga. If you take measurements on a rusted area they need to be well away from the corrosion. The corrosion is actually going be "sloping", from the original thickness area down to the thinner corroded area (at it's worst where there's actual rust through). It can actually extend back from a hole of badly corroded area more than most people realize.
You do realize that each standard gauge has variances, right? It is not one fixed measurement. Standard 18ga sheet is a nominal standard of .0478, but has a min and max tolerance between .0438 to .0518. I've had no problem cutting and forming 18ga. It is more difficult, yes. That's to be expected though. I wouldn't want something in my car, especially structural, that could be formed too easily. That translates directly into DEforming easily! Now working 16ga...THAT sucks!!
If you are trying to form metal and find the more you work with it, the more difficult it becomes, that's normal. It's called "work hardening". You can overcome this by simply heating the metal and letting it air cool back down (annealing).