I had it done today,
Unfortunately the pics I took turned out really bad, poor lighting and cheap phone.
Its an oil based clear spray.
They start off on the top, going through all the doors (inside the box sections) , hinges, joints ect cleaning it up with the airline first, and a wire brush where needed.
Then around all the engine and bulkhead
Wheels off to use the wire brush to get some flaky stuff off, then all blown off with an airline.
Once cleaned up, it gets absolutely soaked in the oil, in all the cavitys through the holes using different spray attachments, went over it all a few times to make sure nothing was missed.
Run a long hose down the chassis with the 360 degrees nozzles, he did it quite a few times to make sure the inside the chassis was properly coated.
Personally I think it's well worth it,
I prefer this over the usual slapping a lot of underseal like treatment on.
Bare in mind though, my chassis and body is mint, if you have a crusty one you really need to clean it all up first.