on the trailer frame you might try some 'chemical paint and rust stripper' I don't recall the brand name I used... it was a very jelly like and off-white in color, came in 1 gallon can.
Yard Friendly - Probably not. But I use to use the stuff to strip steel CO2 cylinders and it worked Great. Brush it on, let it dry, wash it off. Dry and Paint...
I'm sure flapper discs on an 9" angle grinder would work as well, but the size of your new toy would make that a WHOLE LOT of work!
Could turn the cleaning project into a party... Invite the gang over, hand everyone a beer and a sanding block and tell to go-for-it! Hmmm.... Maybe start with a couple Kegs and a case of 16oz Solo cups... That's a BIG BBQ!!!
Whatever you do, Take Pictures as you go!!
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