Suppose you need to keep siblings, roommates, children, or even friends with wandering hands out of something, but you can’t use a lock. Maybe you’re worried they’ll find the key, maybe you need something that has no metallic parts, or maybe you’re a kid and your parents are worried you will lose the key.

This is the kind of thing numbered security seals are good for… but they’re expensive. (The cheapest I’ve found are roughly 50¢ each in packs of 100 or more)

…so I decided to do a little experimentation and came up with a cheap-but-reliable solution for homemade tamper seals that’s so simple and safe that even children can do it.

Instructions

Step 1: Making the seals

The most important thing about a security seal is that it’s unique, so it can’t be replaced, and either can’t be reclosed after being opened or you’ll notice if it is.

To satisfy these requirements with paper, we need to print or write something on the paper which other people can’t reproduce and which will look wrong if someone cuts and re-glues the seal.

If you’ve got a printer, you could print out a random image from the Internet, delete the file and empty your browser history, then cut strips from it to use as seals, but Inkjet ink tends to cost a fortune and most people don’t have laser printers.

What I recommend is this: Cut strips from a piece of paper, then sign and doodle all over both sides of them so any cuts or complete replacement will be obvious. (You do it on both sides, so that you can easily see if someone mends a cut by glueing another piece of paper to the inside of the loop.)

Don’t forget to use your scissors to make the ends of the strip round, rather than square, so it’s harder for someone to attempt to pick at the joint if they want to try to peel it apart.

Step 2: Applying the seals

To apply the seal, you just need to pass it through or around whatever you want to seal (eg. cabinet handles) and then glue it into a loop. (a tight loop, if you’re securing knob-shaped cabinet handles, so it can’t be lifted off without breaking it.)

If you want to seal a box, try gluing several strips together into a shape similar to the ribbon on a Christmas present.

It’s the details which make it tamper-evident:

Get a bottle of Polyvinyl Acetate glue (A.K.A. white glue, school glue, PVA).
Apply a enough glue to one end of the strip that, when squeezed, it won’t leave unglued corners.
Close the loop and squeeze the ends together for at least five seconds, as hard as you can.
Wait at least 5 minutes, but ideally 10 minutes.

Step 3: Making the seals tougher

Supposing you’re trying to keep really clever people from sneaking access, there are two more tricks you should get an adult to do:

Paint the glued part with clear nail polish to waterproof it, so nobody can try to invisibly open the seal by steaming the glue. (This also makes it harder to try to pick apart.)
Cut some diagonal stripes into the glued spot using a utility knife to absolutely guarantee that any attempt to peel apart the glue will result in obvious tearing.

The “Why”

I was very specific in my instructions, because I actually did a lot of testing. If you want to repeat my tests yourself, here’s what I did:

Goals

A successful seal must not have the glue fail when pulling on both ends of the glued joint.
A successful seal must show obvious damage when someone tries to pick and peel at the glued joint.
A successful seal must have the paper visibly fail before the glue if a solvent is applied to the glued joint.

Methodology

Cut a bunch of paper strips, approximately 3″ long.
Glue pairs of strips into longer strips, using various test glues.
On each glued strip, write the time the process was finished, so drying time can be considered.
For each drying time tested, grip the ends of a test strip and pull evenly outward as with a Christmas cracker.
The test is a success if the paper breaks outside the glued patch. (If it fails far enough from the join, repeat until there’s nothing left to grip)
For each drying time tested, attempt to peel apart the two strips of paper.
The test is a success if the paper is visibly damaged in a manner that wouldn’t be hidden by glueing the joint back together.
Repeat each test at least once, to account for variations in the process.
Prepare another set of test strips and allow the glued spot to soak in a drop of mineral or vegetable oil for 30 minutes. Repeat the tests.
The glue passes if it wasn’t weakened by the oil. (In my tests, the breakage still consistently occurred in the dry portions of the paper, indicating that the oil had not weakened it significantly.)
Prepare another set of test strips and paint the glued patch thoroughly with clear nail polish. Repeat the tests.
The glue passes if the solvent in the nail polish didn’t weaken it. (I have already confirmed in previous experiments that nail polish will waterproof paper without weakening it and, in these tests, the breakage still consistently occurred in the un-painted portions of the paper.)

Results

I performed these tests with the following dollar-store adhesives:

Double-sided tape: I tested two different kinds of foamless double-sided tape. Both were too weak to be suitable, losing their grip on the paper during the pull test with barely any visible effect.
Roll-on glue tape: This contact adhesive passed the pull test, but was not tamper-evident in the face of picking and peeling at the joint and was so thoroughly weakened by the oil test that it was trivial to pull open without harming the paper.
Non-frosted Scotch/Sellotape: From personal experience, I know that the frosted variety is meant to be possible to peel off without harm if you’re careful. I tested the un-frosted kind and, while it just barely passed the pull test, it was too easy to peel off without evidence. Also, it was severely weakened by exposure to oil and I suspect this to be a trait common to all readily available contact adhesives.
Roll-on glue tape, plus Scotch/Sellotape: Didn’t perform significantly better Scotch/Sellotape on its own.
Glue stick: Passed the dry and oil tests, but whether it passed the peel test depended very heavily on exactly how I applied it, so I can’t recommend it. It also got severely degraded by the solvent in the clear nail polish.
White/School/Elmer’s Glue (PVA/Polyvinyl Acetate): I was surprised how well this performed… though I probably shouldn’t have been, given that it’s used as a bookbinding glue. PVA dries quickly enough that, after 5 minutes, the still-damp paper next to the joint breaks under test and, after 10 minutes, the paper is back to normal. It is so resistant to peeling that the paper tends to tear, and neither the oil nor the nail polish solvent had a measurable effect on the joint’s strength.

I did, however, notice that peeling at a corner produced damage that was easier to overlook… thus my recommendation to round off the ends of the strip before gluing.

I would have moved on to testing other avenues, such as hot glue, super glue, contact cement, and so on, but I don’t need to test them to know that, at best, they’d match the performance of white glue for this application. (They’re harder to work with, more expensive, no faster to dry, and PVA already produces a bond stronger than and just as water-resistant as the paper around it.)

(I did, however, test super glue on some scraps of projector transparencies meant for black-and-white photocopiers. It passed the pull test but was trivial to peel apart without leaving evidence of tampering.)

UPDATE 2018-08-15: After I realized how effective lighter fluid is for safely getting tape and other adhesives off books and boxes without damaging them, I ran another round of tests.

Lighter fluid does not appear to significantly weaken white glue. However, since my initial tests, I’ve switched to a new bottle of glue and it seems that the resistance of Polyvinyl Acetate (PVA) glues to being peeled off will vary. (I’m not sure whether it varies between batches or just between brands.)

As such, since the quality of your white glue cannot be guaranteed, I now strongly recommend the “making the seals tougher” steps be always used (a coating of clear nail polish to seal the seam and some cuts to improve the chances of tearing when trying to peel it apart), with the use of chevron-shaped (< and >) cuts to ensure there’s no angle one can try to peel at which will be completely parallel to every cut.

Home-made tamper-evident security seals for kids and adults alike