The reality is that each product is different in terms of its drying process as well as its chemical composition. Environmental conditions can differ greatly and will affect the recommended coating times. Companies provide application instructions based on perfect temperatures (23℃) and no wind, but we all know that real conditions can differ greatly.

Some products require wet on wet application. This means that the application of the next coat must be applied while the previous coat is still wet. This is quite common with products like water repellents. Imagine what will happen if you try to apply the second coat after the first coat has dried. The first layer may end up repelling the second layer! Or when working with a cement slurry, the second layer will bond better if the first layer of the cement is still wet.

There are other products however (like membrane forming coatings) where you should let the coating dry and be tack free before you can apply the next coat. Trying to apply a coat over a wet membrane could end up damaging the first layer and lead to all sorts of problems like solvents being trapped within the system.

(Here is an example of a wet-on-wet application gone bad! The second coat started to cure but there was still solvent trapped in the first coat)