Where is best to buy 80% phosphoric acid best for chassis rust removal?

Good evening. I have recently purchased a 1974 series 3 88 and am doing some remedial restoration work. The clutch needs doing so I have removed the floor and bench seat to prepare for removing the gearbox. I now have a great opportunity to rust proof some of the chassis. I have read a few threads on here about rust eating with 80% phosphoric acid before proofing. I have found this stuff: https://www.onbuy.com/gb/p/phosphor.../?variant=84536060&volume=5-litre&strength=85
It seems like good stuff to me and I wonder what you guys thought?
Thanks in advance for your replies.

Damn site cheaper to use a heat gun and a scraper followed by a wire wheel.

Its what you can use to treat the metal after the scraper, wire brush.

I just buy small amounts of eBay. You don't want to use it at 80% either. Curiously it works better when you water it down a bit

Thanks for your reply. I'll probably go for the scraping option for now followed by a good coating of wax oil. With the intention of doing the proper strip-down job in a couple of years.