Can you and have you removed polyurethane without damaging the original paint on a bike?
Why would you remove it? The whole idea is to preserve it as you have it. So that means you've done your little wd40 #0000 rubs, your oxalic acid bath, and you want to stop there.
BLO is boiled linseed Oil & thinner correct?
Does it Not dry out & evaporate? Hence the reason it has to be a repeated process?
The cycle truck OP posted he states
"What would you guys recommend as far as preserving paint/patina?"
That is Every Reason I suggest this.
Now I haven't looked into removing polyurethane on this frame I speak of because I have had no desire to change it. It has not had no valuable paint & I intentionally took it to bare metal before making it rust the way I wanted it.
No smart ass offense meant to anyone but why keep rubbing the same old technique when you can finish it once 🤔