Horological repair, restoration and machining
Different materials expand and contract at different rates with varying temperature and pressure. Watches are designed so the expansion and contraction is kept in check with the use of gaskets. If the watch is exposed to extreme temperature or pressure changes (i.e., hot tub, or diving into water) the expansion or contraction can happen at a rate that would leave a gap between materials, better known as a leak. Properly serviced watches will stay water resistant under normal temperature and pressure changes. It is abrupt or extreme changes that can cause the breakdown of water resistance. Gaskets and materials wear over time and are subject to UV & chemical corrosion (examples - hairspray, perfumes, soap, chemicals, pools and salt water). When all or one of the materials break down it leads to a water resistance problem. Regular professional servicing and replacement of gaskets can keep your watch at its maximum water resistance.
Water resistance - Materials