Try to avoid the acid ha ha. Another option, you can maybe also try a slightly larger hex or a torx bit + force to dig into the rust and remove it, then just replace the wrecked bolt. That's worked for me in the past.
Don't know if you can get o-rings the right size at bunnings. Should be ok for a few times, they just aren't designed to be removed very often (mine crapped out after about 3-4 removals so I got a replacement oem from edge).
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