The best adhesive you can use is a product called Channel Bond, it's specifically made for Polycarb to Polycarb bonding and it's a one part system so no need for a gun, tips, cardridges etc., you just pour back in the container when you're done. Using a Weld-on 3,4, 16 won't do it right. I bond huge polycarb faces for electric sign faces and this stuff works wonders. You can also use it to bond acrylic to polycarb. The only place I know you can get it is here, it does ship hazmat ground uness you can find a place that has it in stock so you can avoid the hazmat shipping fee
http://www.thesignsyndicate.com/foru...-now-shipping/