I work a lot with cardboard. It's a versatile media that's cheap (or free), readily available, and recyclable. One drawback with cardboard, however, is that you can see the corrugations on the side. Of course, you can always work these into your design, but in cases where you want to hide them, it's handy to be able to fill them in.
I tested 5 filling products on scraps of corrugated cardboard and listed the pros and cons of each below to help you decide which might work best for your particular craft application. Play the slideshow below for a quick summary
Silicone Caulk is easy to find and easy to use but it can be a bit of an eye irritant sometimes. I personally hate the smell, but the ease of use still has me reaching for it sometimes. If you’re going to try this out,, make sure you pick up a caulk that says it is paintable - some of them aren’t.
Hot Glue cures really quickly and most crafters already have this in their arsenal. It’s awesome to be able to move on with your project with almost no dry-time, but you can’t easily smooth this stuff once it’s set. Hot glue as filler is super strong when it cools, but I find it very difficult to paint over.
Wood Filler is inexpensive at your local hardware store. It’s non-toxic but the water in it can cause paper fibers to expand and warp. It can shrink and crack when it dries, but I avoid this by gently compressing it into the gaps with my fingers. You can sand it smooth when dry and it takes paint really well.
Spackle or drywall mud is even more affordable than wood filler. It cleans up really easily and takes paint really well, but I do find it has a higher tendency to crack than wood filler. Don’t let that stop you from experimenting with it though - spackle is great for so many applications in the craft and makerspace if you don’t like it for a filler, it’s still a great product to have around!
Cornstarch and Glue is a great emergency option if you don’t have anything else on hand. This stuff can be a bit messy to apply but is super strong when dry.