Would the Sentinels be metal, as that would easily make them so vulnerable to Magneto? Wouldn't whoever made them try to avoid using metal?
I recently saw that 90s cartoon episode where the Sentinels were made of a ceramic or plastic substance. They attacked Magneto on a ship and he used the metal of the ship as his weapon, like firing rivets through them.
I also like to theorise that the Magneto helmet from First Class wasn't metal in its original design, and that's why Erik didn't/couldn't directly lift it off Shaw in the submarine battle. It's worth noting that in the comics, the mutant Fantomex uses a mask made of ceramics (not metal) to block telepathy. Maybe Fassbender's Magneto puts a metal coating on to the helmet for the new redesign in the final scenes of First Class, so he can take it on and off using his powers....