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
I have something a little special for you this week. I have wanted to start posting more tutorials and how-to’s, and this is a good start. This is actually a mini-tutorial because when I was transferring the pictures from my phone to my computer, something went wrong and several of them were lost. Since this is only my second attempt at a tutorial, if you have any questions, or you need a little help with a particular step please let me know!
What You Will Need:
I forgot to take a good “before” shot of the box I was using, so just ignore the fact that I have already covered the two ends!
First things first, you will need to take several measurements. I made separate lists for the interior and the exterior since I was lining the entire box. If you are only covering the outside of the box, your job will be even easier! Most of my measurements for my two boxes were pretty much the same for the inside and the outside, but it all depends on the thickness of the cardboard.
Measurements You Need:
I decided to use my rotary cutter and mat for cutting out the different paper pieces. It was much faster. Using the interior measurements, cut out:
Repeat using the exterior measurements. I added about 1/2 inch to all of my measurements for the sides. That way I know I will have enough paper to overlap if necessary. It is far easier to trim down than to have to cut out a whole new piece! I had two pieces measuring 6 x 12, two pieces measuring 6 x 10, and one measuring 12 x 9. (Tip: Save dull rotary cutter blades and write “paper” on them. Even though they no longer work on fabric, they cut paper beautifully.)
Before you start mod podging everything, make sure that the bottom piece of paper easily fits into the bottom of the box. I recommend having it slightly smaller than the box bottom just so that it will slip in smoothly later.
After you have cut out your pieces of paper for the four sides and the bottom, start painting an even layer of mod podge over the back of one of the smaller pieces.
Once covered in mod podge, the paper will become pliable and easily be able to slip into to box. Since this is the interior of the box, make sure the edge of the paper lines up evenly with the top of the box. I made sure to have plenty of overlap along the bottom and edges. To get a nice crisp corner, you may have to make a few small snips into the edges of the paper. Take your time and try to have as few wrinkles as possible.
Repeat the process with the opposite short side, and then move on to the longer sides. All four sides should be covered in paper before putting in the bottom piece.
All the sides are covered! I decided to put a thin layer of mod podge on the bottom of the box to insure that all the little edges would stick completely. I still painted the back of the bottom paper with mod podge, so you could totally skip this step. Carefully lay the paper in the bottom, smoothing out all the wrinkles, and making sure it looks crisp and even. If you double-checked the size of your paper earlier, you shouldn’t have any trouble getting it in now.
And now we move to the outside! I wasn’t quite sure how I wanted to cover the top edge of the box, but I think I would do it differently next time. For this box, as I covered the exterior of the box, I let it overhang at the top by about 1/4 of an inch. Then after I finished covering the box completely, I dabbed some mod podge along the edge and pressed that little over hang down. It didn’t turn out as smooth and polished looking as I would have liked, but it worked!
The outside is much the same as the inside. Paint the back of the paper with mod podge, carefully line it up on the box, and press it out smoothly.
To get your corners nice and crisp, clip into the corner of the paper after it is pressed on to the box, like so:
Next, press one side down:
And then the other:
Instantly beautiful corners! Repeat for the other sides. For the final piece of paper, I actually covered the actual box bottom with mod podge. That way the paper wasn’t as floppy, and I was able to lay it down exactly where I wanted it.
And you’re done! Step back and admire your handiwork.
And then fill it with all the things! It works great for holding yarn, picture books, toys, various craft notions, and what ever other small items need a new home.
I’m using mine to hold my stash of zippers, elastic, and velcro. Now that I have a pretty box, I can store them on the shelf instead of under the bed! It makes it much easier to access them.
That’s all from me today! I hope you enjoyed the mini-tutorial. Hopefully I can come up with a few more to share over the next couple of months.
Have a jolly day!
~Emily