Painting cardboard is not particularly difficult as long as you understand the fundamentals of paint preparation. Cardboard is really just heavy-duty paper; however, unlike the sheets of paper you can paint water colors on, bare cardboard isn't especially suited for lasting painted finishes. To promote a lasting finish, treat the cardboard like bare wood. Apply a coat of bonding primer that will contribute to better adhesion and a longer-lasting finish.

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Wipe dust from the cardboard using tack cloths. Skipping this step could lead to peeling due to adhesion difficulties.

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Set the cardboard on a canvas drop cloth. You may also use a cheaper plastic drop cloth; however, these tend to allow paint to pool.

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Coat the cardboard with latex primer using a polyester paintbrush. Apply a light coating to prevent ugly sag marks. Let the cardboard dry for two hours.

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Wash the brush in the sink or bathtub, using running water only.

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Paint the cardboard in the same way you primed it, using the clean brush. Let the cardboard dry for two hours. Touch-up any skipped areas if necessary.