I applied 3 coats of varnish, but the surface continues to be rough. How do I stop it from scratching, make it smooth?

Not infrequently I've come across this idea that furniture will feel and look good just by applying successive coats of varnish without doing anything else. Unfortunately there are also very good carpenters who treat the finish superficially, losing sight of the fact that the finish, along with the design, is the first to be seen. And it shows in the finish, because you only have to touch the surface of a piece of furniture and it scratches you to immediately give up the idea of buying it. This is as true of solid wood or veneered panel furniture as it is of MDF. To make the surface smooth and pleasant to the touch, it is not enough to sand the wood; the sanding of the intermediate layers applied must also be done. The person asking the above question was a beginner and was afraid that sanding would completely remove the applied layer. By using the right materials, not only is the primer or varnish not totally removed, but it will be nice and smooth to the touch.

Why does the surface feel unpleasant to the touch

To understand the phenomenon we need to see what happens before the application of finishing materials. Surfaces are sanded to make the wood look and feel good, but also to open up the grain so that the materials to be applied can be absorbed. This absorption depends on colouring, aderation (bonding) of varnish layers and wood protection. Sanding with a certain grit of sandpaper allows us to control the absorption so the wood will be evenly stained and the top coat will cling to the wood and resist mechanical aggression, continuing to protect it.

When it absorbs material, the wood grain rises. The phenomenon is more pronounced if the materials are diluted with water (water-based varnishes or lacquers). Water is part of the 'being' of wood and its deep absorption is natural. Raised fibre is coated with primer/lacquer/paint and becomes stiff after drying. To restore the surface to a smooth finish, sand it with finer sandpaper (see below) suitable for such materials. Do not sand after the bath unless recommended by the manufacturer. Plain sandpaper is sensitive and sanding may remove the colour.

Another reason for sanding is removal of dust embedded in the film or air bubbles formed. And they feel just as unpleasant when we touch the surface. Dust is the declared enemy of the finish. Film-forming finishes - primers, varnishes, paints - don't dry instantly, and while they stay wet, dust particles settle and become embedded. Dust embedded in the last coat of varnish or paint is removed by buffing. Information about this procedure can be found here.

Air bubbles in the film occur especially with finishing materials that dry quickly and do not allow time for air to escape. Air occurs in the application process (air from spraying, air between brush brushes) or from the pores of the wood. If the wood pores are large and the lacquer applied is viscous, the phenomenon is very visible. During the drying process the air tends to escape and moves towards the surface. In the meantime the varnish dries and small ridges form in the film, which are annoying to the touch but are removed by sanding.

Sanding between layers is also done to remove the effect of orange peel. Even if it is not noticeable to the touch, the lacquered surface does not have a pleasant appearance, and the more glossy the lacquer, the more noticeable the defect. By sanding the defect is removed, the surface becomes flat and the gloss of the varnish is pleasantly visible.

How to sand between coats

Sanding between layers is made with sandpaper with a minimum grain size of 240. For best results, sandpaper with a grit of 280 or 320 is recommended if the lacquer or final paint is matt and over 400 if it is glossy. Best results are obtained if every intermediate layer is sanded. Professional finishes are done with 1-3 coats of primer before the final coat of varnish or paint. Primer sands much more easily than varnish because varnish is the final coat and is made scratch-resistant. Also, a primer coat is thicker than a varnish coat, allowing for intensive sanding and the smoothest, most flawless surfaces. This is how furniture factories achieve perfect surfaces that are very pleasant to the touch.

Products in DIY stores are made to be applied as simply as possible, with the assumption that they will be used at home for personal projects. That's why manufacturers often don't have primer and varnish systems, but a single product, varnish or paint, which is applied in 2-3 successive coats. This does not mean that intermediate coats of varnish should not be sanded. Sanding between coats is mandatory to achieve a nice surface finish. Also for simplicity, sanding sponges can be found in such shops which, like sandpaper, do not have a number but are coarse or fine. For sanding the intermediate layers use fine abrasive sponges.

Sanding of intermediate surfaces is done intensively and only after drying is complete. The sanding must result in a fine white layer (coloured, in the case of coloured primers or paints) and the paper must not become gummy and clinging. In this case it means that the material has not been well dried. After sanding, the resulting dust is removed with a dry, soft cloth.

Manual sanding can also be done with sanding machines. These are machines that sand by rotating or moving parallel to the side of the machine. In my view, the latter are more suitable, especially for a beginner. With the others it is possible that the sanding marks remain visible in the form of circles. Use sandpaper with the same grit as above.

The quality of the final varnish or paint film depends on how the sanding is done, both of the wood and the intermediate layers. Ignoring these steps will not produce objects you will enjoy touching. Sanding is a very important part of wood finishing and it is a good idea to find out about mistakes that are made in manual finishing and how to choose abrasive materials for wood sanding. If you have any additions, questions or doubts, please leave them in the space below. I will certainly reply.