disclaimer: I do not own a 550.
But I would only use wd40 to clean the machine up. I'll wager that rustblast thing is pretty similar to wd40.
You don't want to lubricate the machine with wd40.
So what? did that thing wash up on the beach by your house? 
Don't know if rustblast is the same as Evap-o-Rust, but I know WD40 is like water compared to that stuff.
I used it on some extremely rusted tools.Takes half a day to get the majority of rust off. For large items like this, just soak some paper towels in it, and wrap around the rusted parts. Keep checking it. Once the rust is able to be wiped off, remove the towels and clean off the Evap-o-Rust. Leaving it on too long causes increased oxidation which turns metal black. Harder to clean after that point.
I love the stuff.
One thing I have yet to try is electrolysis. I started collecting scrap steel and iron to use for it. You just need a DC 12V or less low amperage power source, like an automotive or cellphone battery charger, a bucket or tub large enough for the part to clean, some copper wire, water of course, sacrificial scrap metal as the anode, and some chemical to allow the water to conduct the electricity like Arm & Hammer Baking Soda soap powder or straight baking soda.
Looks to work quite well. I'll be setting this up soon to get the rust off some benchtop power tools: grinder, drill press, etc.
I've seen if you want to do large parts, you should have several pieces of sacrificial metal wired together to increase the anode "attraction". The anode/s need to be as close to the part being cleaned as possible without touching. Some take a 5-gal bucket and use wire to attach 6-8 lengths of rebar vertically to the inside of the bucket, then attach wire between each rebar piece.