A Mexican recipe for home made glue, probably the cheapest reversible and multipurpose solution to your problem: (walls won't stop the peeping toms, but dirt cheap Mexican engrudo will!)
A traditional recipe for a non-toxic, biodegradable, DIY adhesive compound
- 100g (~3.5oz) wheat or corn flour
- 1 liter (~1/4 G) of cold water
- 5 cc (2 tsp) of vinegar (or 2 teaspoons)
- Optional: vegetable coloring of your choice.
About 1liter (1/4 Gallon) strong natural methylcellulose paste adhesive, non-toxic if accidentally ingested.
In a pot, dissolve some of the flour in a little water until there is no lumps. Add some more water, then more flour untill both ingredients are fully mixed with no lumps (if you try to do it all in one go, you'll get a lumpy mess)
Bring the preparation to a low heat and stir with a wooden spoon. Continue stirring until it begins to boil, and at that point, remove from the heat. Let cool, add the vinegar and mix.
If desired, add a few drops of vegetable coloring of your choice.
Without coloring, the end product will be an off-white tacky paste, (yellowish color approximating the hue of the flour used). When applied to glass, it will behave as a light diffusing, translucent coating.
Handling, storage, shelf-lifetime, and safety.
For this project, probably the best result will be using some
sort of light wide woven fabric, such as the emergency bandages you can
find on any first aid kit. A decorated tablecloth is also great idea
(see pictures for what to expect of the finished project)
You can also use paper (very thin) for translucent effect or even
leaves and cutouts such as depicted on other answers in this thread.
You'll need enough to cover the surface of the glass to be privacy-
enhanced. Just soak the bandages or paper on the engrudo, saturating
them. Then apply and spread over the glass.
Use a wide paintbrush to apply, or, if you know how to handle them, you > can use a spatula to even out the fabric or eliminate bubbles.
Applying first coat on glass, applying a decorated tablecloth, creative application of cheap old newspapers on glass:
Let it sit on a well aired room. Curing times are comparable to commercial white glue. May vary depending on the grain of the flour used (coarse = slower fine = faster) but YMMV.
When done, you can store the unused engrudo on the fridge for about a week. After that, dispose with the rest of your biodegradables. Remember to label it for storage while in the fridge. It's not toxic, but not particularly tasty either.
Guaranteed to keep the baddest hombres from peeping if properly
Bonding materials details
Excellent for everyday tasks requiring adhesives. will bond strongly on Paper, cardboard, wood, and particle boards, calcium substrate ceilings and porous stone or masonry.
Not so good for bonding plastic, synthetic materials or smooth non-porous surfaces such as glass or walls coated with acrylic enamel. On these surfaces you may need to reapply after a few years (therein lies the reversibility of this project)
Engrudo will perform competitively against similar fossil fuel based solutions (I'm looking at you, white glue) on chores such as furniture and upholstery repair, wallpaper application, detailing creative art using wood, paper or cardboard, etc.
All this with the added advantage of being non-toxic, biodegradable and fun to make, plus it's dirt cheap.
You may have all the required ingredients and tools already laying around in your kitchen.
To reverse your privacy-enhanced glass, just tear off the fabric using a spatula or knife to remove stubborn residues, specially where glass meets masonry or wood. Most of the cured adhesive will just fake off the glass when pulling the fabric. This will be relatively easy and leave no permanent stain or mark because Engrudo will bond poorly with glass.
Why it works, science
By heating the water and flour mixture you are creating a cellulose rich saturated colloidal suspension, When you ad vinegar, it w will react with the acetic acid. This chemical reaction will yield a dense methylcellulose solution having specific shear thinning viscosity and a high wet adhesive tack.
It bonds particularly well with other cellulose rich materials, since cellulose will form long interwoven molecular strands, that extend the existing strands on paper or wood, effectively "fusing" the materials in a process analogous to that of arc welding in metals, or the action of cyanoacrylates on resins and plastics (superglue!)
Additional facts, fun, trivia
A slightly lighter mixture of engrudo (diluted with a bit more water for larger yield, increasing absorption on paper), Is still used to soak printed kraft paper posters on it, and then applying to walls/masonry on public buildings for advertising purposes.
It's applied using a long paintbrush resembling a broom, taking the just soaked on engrudo poster and spreading it out on the wall surface. Engrudo will bond with the masonry, trough the porous paper and the end result is a nearly indestructible advertisement. Wooden telephone posts are used for this purpose too, with subsequent applications taking advantage of the substrate created by older layers of engrudo soaked posters. (Just apply the new poster on top of the older ones.)
I've seen maintenance crews sandblasting old advertising affixed to churches and public building walls using this technique.
Examples of nearly indestructible advertising: