There are several trains of thought on how much glue / solvent cement a plastic pipe and fitting require. Some advocate slapping it on in huge quantities and not caring about the mess it may cause, whereas others prefer a minimalist approach to try to keep the joint looking as tidy as possible, whilst at the same time, cutting costs by using less plastic pipe glue. So, which is the right aproach? Firstly, let's recap how the glue / solvent cement works. Glues like bostick, pritt etc. adhere to both surfaces (hence they are called adhesives)and then dry out, forming a bond between the surfaces. PVC and ABS pipe solvent cement (technically it is a solvent cement rather than a "glue") works in a different way. The solvent in the liquid, melts the outer layer of plastic on the pipe and fitting. When they are then pushed together in a nice tightly fitting socket, the melted surfaces, mix together and then, as the solvent evaporates, reform to solid plastic. This therefore gives a continuous plastic layer between the pipe and fitting, which results in a strong and sealed joint.

Adding too much cement to a pipe and joint does not have any derogatory impact on the sealing of the joint, however excess glue which comes out the joint, if left in contact with the plastic, can, over time, weaken it, and reduce it's pressure rating. So, if you're a slapper, wipe the excess off! An advantage of slapping the cement on, means that you have an extra couple of seconds to adjust the position the joint. Take care though as any repositioning carries the risk of tearing the reforming plastic skin in the joint. Any reposition is best done as early as possible.

Adding too little cement has the obvious risk of not covering the full surface of the pipe and socket. This gives you a double whammy of having reduced strength to resist pressure and also the risk of areas of the joint where both surfaces have not had sufficient solvent on to melt and then re-bond together.As there is less solvent in the joint (with less cement) the joint dries out quicker and so is more prone to damage if any repositioning is carried out. Basically it starts to set that little bit quicker, so by repositioning / turning, you increase the risk of causing tears in the joint, which will manifest themselves in the form of leaks.

There are different types of solvent cement on the market, some of which are as thick as porridge and others that are as thin as water. Some users are of the notion that the thick types of glue have plastic melted in them. This is NOT the case. Thick cements are more suitable for large pipe diameters and fittings where it takes longer to cover the joint. For smaller pipe and fittings below 160mm (6 inch), the lower viscosity glues are more suitable.

So how much solvent cement is it right to use? In my view, you are best to use slightly too much than slightly too little. This is easier with the thinner cements as excess just runs out the joint and of the pipe even before you assemble.