first off what is the printer doing? the encoder should be left alone if the printer is acting okay. or very lightly cleaned here and there.
white piece of paper (kind of stiff), magnifying glass, flashlight.
print this file (single pixel vertical lines in a row) set to bi-directional in sections longer and longer (1, 2, 3 copies etc). you will see in the print when the return staggers to the right (this is the carriage missing steps and trying to count backward further to the right). go to the point that the stagger began and look with the white piece of paper behind the encoder strip with the magnifying glass. it is a PET laser copied image (microfilm). on most every printer, the image is reverse printed so the side facing you is not the printed side. the unprinted side of course can be cleaned more aggressively than the printed side.
there's a lot of advice saying use isopropyl only. i've used straight acetone when i was about to give up on a little speck. i go quickly and dry it off manually with a new q-tip (i don't recommend this. if you ruin your strip, don't blame me. but i have done it successfully).