LG has said repeatedly that the LG G5 has a metal alloy body with cleverly hidden antennas. The company didn't really go into detail about how that was achieved, except to say the process is called micro-dizing. It seems that micro-dizing may have something to do with plastic because a teardown video from YouTuber JerryRigEverything reveals what appears to be a plastic coating on top of the metal frame.
As you can see at about the one minute mark, there's some sort of polymer on the G5 that can be scraped off. Below that is aluminum (or a similar metal). Coincidentally, I remarked in the AP chatroom earlier that the G5 feels a lot like plastic for being a metal phone, and then this video popped up. So yes, you can tell something is unusual about the device.
Presumably the plastic layer is there to cover the antenna bands and give the device a more uniform look. However, the entire body appears to be covered in this material. We've reached out to LG to see if we can get clarification on this. The company has previously pointed to the PR that talks about the hidden antennas.
LG has come out with an official statement on this whole fiasco. The material you see peeling off the metal frame in the video is not technically plastic, but primer. You'd usually use primer when you are planning to paint something, but it would seem in this case it's part of the process that hides the antennas. Primer is usually some sort of synthetic resin mixed with solvents and other materials. It doesn't feel entirely unlike plastic, but it's technically not plastic. I don't know that it matters too much what the material actually is in this case. It feels unusual and not like metal. People will pick this phone up and think it's made of plastic even though it's just covered in "primer."
LG has posted an additional statement on the material controversy, which you can read below.
Contrary to erroneous reports online that the LG G5 body is composed of plastic, LG would like to state unequivocally that the uni-body casing of the G5 is composed of a special aluminum alloy, LM201 (Feb. 2016 patent pending). LM201 was developed by LG in partnership with the Korea Institute of Industrial Technology for use in high-end sports cars, aircraft and consumer electronic pducts where durability, rigidity and lightness are required. As a die-cast metal, LM201 was determined to be most appropriate material to accommodate the curves of the G5. After the die-casting is complete, the insulating antenna slit is applied directly to the aluminum casing followed by a coating of primer which provides additional insulation and enhances the surface profile for the “microdizing” process, where pigment containing tiny metal particles is applied directly to the primer to provide a smooth and durable finish. While both anodized aluminum and microdized aluminum will scratch if enough pressure is applied, in the case of the G5 the gray primer beneath the pigment layer may be mistaken for plastic when the coloring is scratched off. We want to reassure our customers that the uni-body of the LG G5 is advanced aluminum alloy, not plastic.