Oh come on IO, the rental scene is just as concerned about corrosion as you and I, these are their bread and butter and an airframe out of action for corrosion is not earning money. You really must have had a bad time with a school somewhere to be so down on them ALL the time!!!!
Truck deicer and the aero shell stuff is identical, just one has 'paperwork' and the other does not. 20l drum about £20!
I use a B&Q pump to put it on and it's worked fine for me for years and nothing has rotted off my aircraft!