A blown up panel from one of the pages of El Indio. This is a rather difficult panel to restore because of the printing error on the lower part of the artwork, specially the 2nd speech balloon. Removing the yellow discoloration was actually quite easy in comparison.
Fixing the printing error entails scanning the artwork at a very high resolution. 1200dpi was recommended, but I don't think my computer can handle it, but I think 800 dpi RGB would do just fine.
Then one would go in a slowly shave, erase the dirt pixel by pixel. The damage to the lettering entailed more than just cleaning, but rather re-lettering. I did not even attempt to try and letter it myself, but I opted to cut and paste letters from either the first balloon or from other pages so that the look of the original lettering could be preserved.