Joining {or sealing} of preformed parts {, e.g. welding of plastics materials}; Apparatus therefor using adhesives {, i.e. using supplementary joining material; solvent bonding} using adhesive tape {, e.g. thermoplastic tape; using threads or the like covering both elements to be joined and placed in a recess formed in the parts to be joined, e.g. in order to obtain a continuous surface}