Fully cured extruded cord is typically cut to length with a straight 90 degree or 45 degree angle scarf cut ends depending on the material type and thickness. The ends are treated with a mixture of uncured rubber and adhesive and placed in a tool fixture to heat and cure the joint. Once removed from the fixture the joint might be buffed but imperfections could remain that can cause leaking in some applications thus recommended for very basic short term non-critical applications. Joint quality can vary as they are produced by hand. Good joint tensile strength can be achieved with Buna-N, Neoprene and EPDM, reasonably good with Viton and only fair strength can be expected with Silicone. ID dimensional tolerances per RMA Table 18, S2 Commercial or better.
Potential excessive adhesive at joint (Exaggerated Renderings to describe potential issues)
Buffing at the joint can yield variable surface finish from the rest of the part