When solvent welding with Upvc, the pipe should be clean from dirt and oils. The solvent welding is a two phase application with the red bottle in Fig 1 being the priming fluid and the blue capped container is the solvent cement.

The priming fluid is used to clean and prime the pipe for joining while the solvent cement is the glue. The priming fluid is prone to evaporation, so keep the lid firmly on the container between primer applications to the pipe.

When priming or gluing the pipe it must be done quickly and with even applications of both products as shown in Fig 2. The primer will be rubbed into both joining ends of the pipe and then the glue is brushed evenly around the two joining pieces. 

Once that is completed then bring the two pipes together and twist whilst joining and hold firmly together for a minimum of 30 seconds as displayed in Fig 3. 

Remove and clean away all excess glue.

This can deteriorate and weaken the pipe.