To clean its mould lines, the most important thing to have is a very sharp blade; the second is to avoid “scraping” the line but to prefer going for clean and straight cuts.
You can use files on surfaces that are more open and accessible, but filing or scraping the surface may create residues that do not fall off like they do with metal and plastic, but remain attached. This is due to the more plastic and flexible nature of the material itself.
If the area to clean is easily accessible, this can be resolved easily; if you are trying to clean a mould line between the legs of a horse instead, this may be a bit more complex.
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