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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?

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up vote 6 down vote accepted

Plexiglas (poly (methyl methacrylate)) is strongly attacked by xylene, one of the solvents in the sealant you selected. I'd expect the sealant to soften the plexiglas and probably also whatever plastic the air conditioner is made of.

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I see. Thanks. Is there a different kind of sealant that you can recommend? –  dB' Jul 16 '13 at 17:59
    
I think ordinary RTV silicone sealant sticks to acrylic, but you should check the label. –  ArgentoSapiens Jul 16 '13 at 18:15
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@ArgentoSapiens hit it on the head -- it is the Xylene in the Seal'N'Peel version of sealant (not silicone sealants in general). I would go with a 100% silicone like GE Silicone II, which I've never had a problem with on plastics. It is fairly easily removed from flat non-porous surfaces like plastic, after fully curing, using a flat razor blade. –  Jacob S Jul 16 '13 at 18:39
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