Plexiglass is a popular acrylic building material with the lucidity of glass yet less rigidity, making it more resistant to shattering. The only drawback to this awesome clarity is that a sloppy glue job looks especially unattractive. However, being too neat and tidy can lead to a weak seal between plexiglass pieces. The key to properly gluing plexiglass is using the right tools, both before and after.
Lightly sand the edges of the plexiglass you plan on gluing. This is done to coarsen the texture so that the silicon gel will have more microscopic nooks and crannies (i.e., greater surface area) to stick to.
Stand the pieces of plexiglass up, with the edges you plan on gluing next to each other.
Place one strip of metal across the upper surface of the plexiglass so that it spans both pieces. Slide the pieces together so there is a half-inch space between them.
Apply one C-clamp to each plexiglass piece, securing the metal strip to both (Figure 1).
Place the remaining metal strip along the bottom edge of the plexiglass. Secure both pieces to the strip with the remaining C-clamps, again maintaining a half-inch gap.
Load the silicon adhesive into the caulking gun.
Apply a continuous line of silicon (with a width roughly half the thickness of the plexiglass) along one of the sanded edges.
Slightly loosen the top and bottom C-clamps on the glued plexiglass sheet.
Carefully slide the glued sheet onto the non-glued sheet until the silicon has squeezed out just 1 mm over the edge.
Tighten the clamps back up.
Slowly push the flat edge of the trowel along the inside and outside of the corner to skim off the 1 mm overflow. Note: do not "scoop out"; just flatten.
Let the glue dry for 12 hours.
Check edges for silicon residue from where you dragged the trowel.
To remove residue, fold a paper towel in half and wet a corner with acetone. Remove excess liquid.
Gently rub back and forth until the residue comes off. Once clean, allow to air dry for 1 hour.