2nd jcheese's suggestion, i think you said you used acrylic, so if a decent cleaning product doesn't do the job then leave it to soak overnight in some. I used to use meth spirits to remove acrylic from models
I wish I had enough disposable income to waste an entire upper receiver.
How is applying some sprays of household cleaner and leaving it overnight too much effort? You actually have to work for things in life, you can't expect them to go perfectly 100% of the time