Consider a related example:
All is good.All are good.
Both forms are correct, but have slightly different meanings/contexts. The first is correct if "all" refers to "everything" (singular); the second is correct if "all" refers to an implied concrete set of things, as in "all (the widgets) are good". This line of reasoning seems to imply the latter form is correct, since "paintbrushes" are concrete things. (And yet that seems to be the less popular form in common usage, judging by Google hits).
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Mr. Tamer Shaaban
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