The undershirt being neoprene was mentioned a while back and was point blank refused. Maybe if it didnt get shot down it could have been found already.
There is now a lot of evidence pointing to the under shirt and pants being a wetsuit. Too much for command staff to simply say the undershirt is not made of neoprene.
By listing them below with pictures and reasoning I think that it will be easier for everyone to digest.

•- First of all there is the comment form Karanje. One of the stunt men who put the suit on daily whilst filming. He says that it was a dyed neoprene wetsuit. Admittedly the actors can be wrong but if a guy who has done many movies says that it was, his opinion carries a lot of weight.
•- It is thought that the trousers and undershirt are made from the same fabric. Again, this is in keeping with a 2 piece hooded wetsuit. The bottom half left black and the top half dyed red.
•- The 2 pictures above show the shape of the wetsuit balaclava on the model as being the same as the premiere picture where the guys lid rides up. Again this looks like a wetsuit. In fact on this photo, without any extra lighting, the red area looks like a wetsuit material.
The top 2 of these 3 pictures are all taken of a screen used suit in LA. The top photo is under exposed which IMO helps to show the neoprene fabric. You can see, without any extra lighting, that it looks to be a smooth fabric. Not wool, not Lycra, not snake effect.
The middle photo is also from the same exhibit, but with lighting or a flash. Now you can see a small texture or pattern but I think that this is more of a lighting trick. The third photo shows the suit from close up at a premiere. Again, the texture looks like a wetsuit. Which brings me on to the next 2 close ups.
These are taken at the LA show on a dsl camera and are zoomed in on the undershirt and neck area.
Again, IMO you can see that when zoomed, the fabric has the lines and texture of neoprene that has been painted and possibly distressed and had some sort of lacquer painted on. Possibly a water sealer to protect the paint when they got washed.
This photo is what I think has caused a lot of confusion. The left side appears to show a textured, almost snake like pebbled fabric. The right side and lower area are more in line with the wetsuit theory. This photo was taken in the same light as most of the others. I think that the lighting is making the fabric look different.
I think that a wetsuit was used. I think that it has been painted red and sealed. I also think that before it was painted it was possibly distressed.
These are only my thoughts and I know this theory has been dismissed but I think the pictures above and the words of the stunt man merit it being considered.
Thanks