I've installed an air conditioner in a large horizontal sliding window, and I've used plexiglass to block off the space above the unit. I'd now like to seal the edges of the plexiglass pane to prevent air leakage.
I've bought a tube of silicone sealant to do this, but I've just noticed that the instructions say not to use it with plastic. ("Adheres to: wood, glass, metal, vinyl, aluminum. [...] Do not apply on plastics.")
The A/C unit is made of plastic, as is the plexiglass (I assume). Is this a problem? Should I not use this sealant around the edges of the plexiglass pane and the A/C unit? If not, why not? Why is the warning there?