If you’re using plexiglass to construct something, you’ll need to join plexiglass sheets together. It’s a good, strong material but you need to be very careful when using glue. If it smears it can look very bad indeed and ruin the object. There’s an art to joining plexiglass sheets together. If you know how to do it and have some practice, the work will look perfect every time. It’s important to have the right tools.
As you’ll be joining the plexiglass sheets at the edges at a 90-degree angle, begin by sanding the edges that will join. You should make sure you use a very fine grit of sandpaper, and be careful where you sand or you can scratch the plexiglass that will be on view. The idea is to slightly roughen the texture of the plexiglass to give the silicon more area to which it can adhere.
Stand the plexiglass sheets so that the edges you intend to glue are next to each other, with the two pieces at right angles. Bend one metal strip into a right angle and place it high up on the plexiglass sheets so it touches both pieces. The plexiglass sheets should about 1/2 inch apart. Use a C clamp on each of the plexiglass sheets, on top of the metal strip, to keep the sheets in position.
Now, you need to repeat this at the bottom on the plexiglass sheets, again bending the metal and using the C clamps to secure it, with the sheets still 1/2 inch apart.
Cut the tip of the caulk and put it in the caulking gun. You need to apply a continuous thin bead of the clear silicone caulk down along one of the edges you’ve sanded. Thin is the operative word. If it’s too thick you’ll find caulk squeezing out when you join the plexiglass sheets.
After applying the caulk loosen all the C clamps slightly. Very carefully push the plexiglass sheets together. Push until a very think bead of caulk appears outside the edges of the plexiglass. Tighten the clamps to hold the plexiglass in position. Use the flat edge of the rubber trowel to flatten and skim off the extra plexiglass. Don’t try to scoop it out, just flatten it.
After the silicon has dried for 12 hours you should remove the clamps and inspect the joint. There will inevitably be some silicon showing. You’ll need to remove this in order for the join to look good. The way to remove the silicon from the plexiglass is with acetone. Put some on a paper towel and wipe away at the silicon. Keep rubbing gently and the silicon will come off. After removing the silicon residue you should leave the joint to air dry for an hour.