What it is and why sanding between coats is important

Suppliers of wood finishing products continually tell their collaborators that sanding between coats is a prerequisite for a quality finish. I even used to say that the products are very good, but they can't work wonders. But what is it, why and how is this sanding done and, more importantly, why is it so important?

Sanding between the layers of varnish is not totally different from white grinding, i.e. that which is done before it is covered with finishing materials, but has its own peculiarities. It is done in order to obtain the best possible surface quality and to ensure adhesion between the finishing coats.

Let's take quality first. When the first coat of finishing material is applied, the wood absorbs this material and the wood grain rises, less if the materials are solvent-based and more if they are water-based. When the applied material dries, this raised fibre becomes very stiff, feeling scratchy when you run your hand over it. For the surface to become smooth, this roughness must be removed, and the removal is done by sanding.

But it's not only high fibre that creates problems, but also dust in the atmosphere or film gassing that can occur when spraying. Gassing occurs when particles of dry varnish from the atmosphere fall onto the wet film and get trapped in it, behaving like an abrasive (you can scratch yourself on them). When sprayed, the lacquer is transformed into very fine particles. A very volatile (very fast) thinner, combined with a working environment where it is very hot, causes some of the varnish to still dry out of the air and fall onto the wet piece and ... ready to gas. Both these varnish particles and the dust particles trapped in the varnish film are removed by sanding.

If more than 2 coats of material are applied it is best to sand after each coat. There are technologies that recommend applying coats by the wet-on-wet method. That is to say, if 2-3 coats of primer are to be applied, they should be applied without waiting for total drying but as soon as some of the thinner has evaporated, i.e. between 15 and 30 minutes after application (no more than 45 minutes in the case of polyurethane products). In this case, of course, no sanding of the coats is done, but it is mandatory to sand this multilayer after it has dried completely, before applying the final varnish coat. The same applies to intermediate coats of UV products which do not cure completely but only gel.

Another property influenced by sanding between layers is the adhesion of the layers of material to each other, where the nature of the materials used is very important. If nitrocellulose products are used, the layers of material will adhere even if no sanding is done. In this case the thinner in the upper layer penetrates the lower layer, softens it and joins it to form a whole. The same is not true for chemically cured materials. In this case, because the thinner no longer attacks the lower layer, a way must be created for the upper layer to 'cling' to the lower layer. And this is done by sanding between the layers, when small curls form in the film, to which the top layer clings. In addition, sanding removes the fully hardened part from the surface of the deposited layer and reaches inside, where the material is softer and the reaction is not complete. It is therefore better to sand before applying the next layer. It is not a good idea to leave a long time between sanding and application (e.g. overnight) because during this time the "uncovered" layer comes into contact with the air, hardens completely and you end up exactly where you started.

Adhesion between layers can be controlled simply with materials we have at hand. One method is with a coin that is passed with its edge over the varnish layer. If there is a white, broad mark, then the layers do not adhere to each other. Another way of testing is by making with a fine caterpillar a grid of x's and 0's on a hidden part of the finish. Over the grid put an adhesive tape (scotch tape) and pull. If more than 15% are removed from the squares formed then the varnish has no adhesion.

Unlike white sanding, sanding between coats is done with fine abrasive sponges or 280-320 grit sandpaper for matt varnishes and 400 and more for glossy ones. The use of coarser sandpaper (e.g. 100-150) is totally inadvisable because the curls formed are very large and become very visible after the next coat has been applied. Also, sanding with a coarse paper removes a much larger amount of primer and there is a risk of getting wood. I have seen in some factories how, out of a desire to make sanding faster, coarse paper was used, ending up removing the entire layer of primer applied. It was a kind of "work in vain" because it would get back to the wood and when the next coat was applied the wood grain would rise again.

In order not to risk getting into the wood even if you use the right materials, it is best to first carefully sand around the edges of the panels and then move on to sanding the central part.

It is also a good idea not to use sandpaper, which is more rigid, for the profiled parts, table or chair legs or other such areas, but sponges or other materials that mould to the profile. This avoids 'burning' of areas, i.e. removing the finishing materials in those areas down to the wood.

Sanding between the layers can be done both manually and mechanically, with special machines with rotating movement or in plane, the important thing is that the sandpaper used is the right one.

I hope you've managed to get an idea of what sanding between coats means and why it's good to do it. There's a lot more to say about this topic, but I think it's already too much for one article.