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Mildew (Mould) can be removed by cleaning with:
a) bleach
b) a mould remover (in New Zealand or Australia this is Exit Mould but the generic supermarket brands work just as well)
c) vinegar
d)...
Epoxy thinners is the only thing you can use to thin down Epoxy paints however for cleaning other thinners such as general purpose and gunwash should do the job aswell.