Projection welding is very easy to maintain if the fundamentals are taken into consideration in the first place. Steel wire shopping carts, wire racks, and other crosswire products are all projection welding. Nuts and bolts are often assemble to other metal parts by projection welding. If you want to produce a part with very little heat surface marks on one side of the panel, then processing a projection on the alternate part and using large electrodes contoured to the finished part would facilitate the goal.
In 1828, the first indoor swimming pool, St George's Baths was opened to the public. By 1837, the National Swimming Society was holding regular swimming competitions in six artificial swimming pools, built around London.
There are pools/baths dating as far back as the Ancient Greeks and Ancient Egypt however I believe there would have been something far more prehistoric than this (albeit far less advanced). The first heated swimming pool was built by Gaius Maecenas of Rome in the first century BC. Gaius Maecenas was a rich Roman lord. The great bath at the site of Mohenjo-Daro was most likely dug during the 3rd millennium BC. This pool is 12 by 7 meters, is lined with bricks and was covered with a tar-based sealant.