Compression fitting

Compression plumbing fittings are fairly easy to fit, and make a secure joint. They're also fairly straightforward to remove later and require no soldering skills. Hence they're popular.

Metal

Copper pipe works well with compression fittings. Chrome plated pipe needs its chrome stripped off at the end to seal well.

Compression joints are designed to be used without any sealant. The nut compresses the olive, causing deformation of both metal surfaces, zeroing the gap and creating a watertight seal. This works well with new pipe.

Its a different story with old pipe. Sometimes it works fine, and sometimes tiny surface defects prevent a metal to metal seal.

Another way to use compression fittings is to coat the mating surfaces with sealant before assembling & tightening. Opinions tend to polarise over whether one should do this never, sometimes or always. When even this is liable to not seal, eg when the pipe is visibly deformed or damaged, packing the nut with sealant as well can make it work. This forces sealant over and into the joint as the nut is tightened - the sealant left behind in the nut does nothing to create a seal.

Copper and brass olives both work fine on copper pipe. Olives must be totally unused. Even very gently hand tightening a fitting with an olive inside is enough to make the olive unusable. Used olives are totally not reusable.

When remaking a joint where the old olive is stuck firmly to the pipe, there is no real likelihood of a bare metal seal being successful. Either cut the end of the pipe off and start afresh, or reuse the olive by packing the joint with sealant, relying on that to do the job.

Sealants used on drinking water must be suitable for potable use.