Question: I am looking to use a piece of wood to lay under my sofa cushions because it sinks to much when you sit on it. I can't afford a new sofa so I was told I could use a piece of wood under the cushions. I need it to support a person 300 lbs- 400 lbs. They are disabled and do a lot of sitting.

Answer: 
WD40 is a good product but it's nothing special for removing rust. There are similar lubricants sold that are labelled "food safe" for use in commercial kitchens. (You're going to wind up washing off whatever you use, and I doubt traces of WD40 would be dangerous at all, but why even take a chance.) 

A lot of food safe lubricants you'll find are silicone-based, I'd avoid those for this purpose. 

Plain mineral oil is sold in pharmacies, and it's one of the main ingredients in WD40 and many other lubricants. In addition to the medicinal uses, it's used on cutting boards and that kind of thing, it would certainly be safe. It should work fine to loosen rust. 

For that matter, any vegetable oil will soften the rust the same way, although it might not be as good as mineral oil. Canola oil was used as an industrial oil before it was used as a food oil. 

If there's oil - whether it's WD40 or mineral oil or vegetable oil - on a heating element, it will burn off when the temperature exceeds the flash point of that oil. You'll want to remove as much as possible simply by washing with soap, dish soap should be fine, then drying immediately.
I use barbeque cleaner and it works wonders especially if you let it soak for few hours.
After you clean off the barbeque cleaner, wash the hobs of with soapy water.
Just make sure you rinse it out very well before you turn the hobs on again.