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I am building sets and props for an outdoor performance. We must use recycled materials as much as possible, and will thus use cardboard a lot since it is readily available. We plan on painting or sticking on some decor to build the props, where they will then be mounted on frames/sticks to stand or to be puppetted by stagehands.

As the performance is outdoors, the pieces will inevitably face the perils of water (tropical climate; high humidity and precipitation) repeatedly due to rehearsals and accidents. I am concerned about the cardboard pieces falling apart from water damage. Most of the cardboard is 1-ply, if that is important.

How do you cost-effectively waterproof large pieces of cardboard with paint on them and protect them from getting soggy? They will be on the order of 0.25 to 16 m^2 in surface area per piece.

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    What kind of paint do you intend to use. The paint itself could be a water proofing (acrylic or similar). Otherwise I think a spray polyurethane or similar would work great. – BrownRedHawk Jun 11 '16 at 18:56
  • @BrownRedHawk You could flesh that out into an answer, and maybe address a couple types of paint. Your answer could help decide that (for Darren or someone else) – user24 Jun 11 '16 at 19:41
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    Paint is only part of the answer though. It would be hard to protect the edges of cardboard with paint. – Matt Jun 11 '16 at 20:07
  • @Matt yes, I am concerned about the edges as they will be cut. The corrugated parts will be exposed. I was gonna use either spray lacquer or cover the face in a glue that cures clear but I am not sure how to deal with the edges. Would silicone sealant work? (The kind you use in fish tanks or windows to join/waterproof glass connections) – Darren Jun 12 '16 at 8:59
  • @Matt probably a mix of wall paint (a lot of coverage needed due to the size) for the background color and acrylic paint for the finer details – Darren Jun 12 '16 at 9:01
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Waterproof paint for the flat parts, tape for the edges.

Any kind of spreadable waterproof fluid will do for the large faces of the cardboard sheets. Paint or glue could both work, but paint is probably more cost-effective. There are many types of waterproof paint (acrylic, enamel, anything listed as exterior-grade).

The edges will be harder to deal with using paint. Even if you managed to paint the edge itself successfully, water could still trickle down the corrugated part to the inner parts where your paintbrush couldn't reach and destroy the cardboard from the inside out. But the edges can be covered using waterproof tape. Duct tape would be best for strength and durability, but if you don't want it to be visible and spoil the image of your painted props, you could use a strong clear tape instead.

See also this Quora article:

There are various ways you could seal cardboard but basically you'll have to cover every part of it that would ever come in contact with water or even humidity, and do that completely and without even tiny gaps.

So you could seal it with waterproof (or at least water-resistant) liquids, or with waterproof tapes (duct tape, clear packing tape, etc), or by using something like shrink-wrap plastic or other solid but flexible plastics, or with flexible metals like aluminum foil, or even just by enclosing it inside a waterproof acrylic or glass display box or dome.

The most water"proof" liquids would be permanent paints (acrylics, latex, enamels) or clear finishes like polyurethane and epoxy resins and even less waterproof ones like thinned-down PVA glues** and acrylic mediums, etc. (Many other pasty, etc, materials that are thicker or not intended as finishes/paints could work too but they wouldn't usually be as smooth or thin.)

** aka permanent white glues, craft glues, etc --Elmer's GlueAll is most commonly used or book binding glue-- and "decoupage mediums" like Mod Podge which are already thinned down white glues

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