I've got a "Green Star" twin gear juicer which I have used to juice a variety of vegetables, but mostly carrots and celery. I always juice the celery last, as that seems to minimize the amount of direct orange staining from the carrots. But over time I have noticed a buildup or "caking" of stains on various surfaces that the juice touches despite cleaning after each use with warm soapy water. The stains are brownish in color and form a "film" that seems to increase in thickness over time.
In the past I have found that once the stain gets "thick" enough I can scrape it off if I have the appropriate plastic tool or even my fingernails, however only on certain surfaces (some are hard to reach or I don't have a suitable edge for the shape of the surface). This has also worn the tip of my plastic cleaning device that came with the unit down to where it's not much good for cleaning any more anyway.
I have recently tried a cleaner that contains oxalic acid, and while this seems to help, the directions say to not leave it on the surface for more than a minute but even 5-10 minutes applied directly to part of the stain doesn't break it down. I have tried soaking the strainer in vinegar for half an hour, but this also was insufficient.
This picture is of the end piece which controls pulp ejection. I did a test where I left a blob of cleaning with oxalic acid for 10 minutes. As you can see it helped somewhat but did not remove the stain:
Update: I have tried soaking in water and after around 36 hours it still isn't loosening up any.
As a note, I also have a similar problem with the glass bottles I store the juice in until I drink it. In this case it's a much thinner film, but it does not completely rinse off and also builds up over time. I used to use a plastic bristle cleaner and it worked presumably because the film doesn't stick to glass as well as it does plastic, but I kept breaking the bottles trying to pull out the cleaner - the mouth is very narrow and it was very difficult to get something that would both fit into the opening and be wide enough to reach all the insides. I'm using a thinner scrubber now but it can't reach everywhere. I tried soaking a patch overnight in lime juice as well as an hour with oxalic acid and neither really made a noticable difference.