Rust repair
 

My supra has quite a large rust hole at the rear quarter panel bottom and upper wheel well(just behind the rear speaker). To make things worse it is quite hard to sand or wire brush the area. I prepped the metal with phosphoric acid and rust primer. It looks quite good.
Now I am considering my options to fil those holes. I could use fiberglass but I doubt I could apply it properly on those areas. I am wondering how that "closed cell" expanding foam is. Would it be a bad ideea to fill the bottom of the quarter panel with that stuff? If I fill that area with that foam it will also be much easyer to fiberglass the area.
Are there any other techniques that do not require welding?