7 rules for a perfect gloss on wood and MDF

Achieving that perfect sheen, in which the surrounding images are reflected as in a mirror, is one of the challenges of finishing wooden or wood surfaces. MDFeven in experienced factories. Gloss is not only about the quality of the varnish but also about many other conditions that must be respected. Each condition has its share in achieving the perfect glossy surface and one without the other does not come out right. I am not a fan of gloss and when asked for advice, not infrequently my recommendation is the matte finish. Because the glossy finish is also very hard to maintain not just achieve. But I have to admit that there have been times when the gloss was so right and perfectly chosen that it won me over. That's why I think it would be useful to have a list of conditions that need to be met to result in a perfect gloss surface.

1. The place where the finishing is done must be dust-free and at a suitable temperature

The biggest enemy of gloss is dust. You cannot achieve a mirror-like gloss and a surface that is pleasant to the touch if there is a lot of dust in the area where the varnish is applied or if mechanical work is being done on wooden surfaces. The small dust particles trapped in the film are much larger, like small craters, because they are magnified by the gloss varnish.

If you have a workshop, find a separate space in which to do the varnish application, different from the one in which the mechanical work is done. If this is not possible, apply the varnish on a different day than the day the machining was done and after you have cleaned the workshop by removing the wood debris. Water the floor with water because water attracts dust particles from the atmosphere. Don't start painting as soon as you have cleaned up. Let the dust settle overnight, and in the morning wet the floor again.

To those in factories I recommend pressurised spray booths. Spraying is done in enclosed spaces where there is slightly more pressure than outside. This prevents dust particles from outside being drawn in. Drying of the film is also done in these spaces because the lake is vulnerable as long as it is wet. Booths can be bought or spaces can be set up where air recirculation is calculated so that there is always a small overpressure inside. The change in the volume of air in the enclosure should not be done quickly because currents occur which produce small waves in the wet film, waves which will influence the final gloss.

Temperature is also important. The optimum working temperature is between 18 and 20°C. It won't be a tragedy if the temperature is a little higher or lower, but if the varnish is applied at 10 or 35°C, you won't achieve perfect surfaces. Also bear in mind that this temperature does not only refer to the workshop but also to the wood or varnish. There is no point in having 20°C in the workshop if the wood or varnish is brought in from an unheated warehouse just before the application starts. A cold varnish or paint will not spread well on the surface of the wood or wood-derived boards and you will not achieve the desired gloss.

2. The surface to which the primer and varnish/paint is applied must be very well prepared

Before applying the finish, prepare the surface thoroughly. It must be very well polished, without dents, holes or other defects. Remember, the gloss magnifies any defects. If applied to solid wood or veneered boards, sanding should be done at the end with sandpaper that is not very coarse because the curls left after sanding may become visible with a glossy varnish on top. You can finish with 180 or 220, even 240 sandpaper. Finer sanding may result in less adhesion of the primer.

After sanding, remove the remaining dust thoroughly with a cloth. If a blower is used, do not apply the primer in the area where the fine wood dust was blown. If using wet rags, do not use water if you are going to apply solvent-based materials but the primer solvent.

If you're going to apply the finish to MDF panels, make sure you have a quality, dense panel with no small holes that eliminate air when the primer is absorbed. This air gets trapped in the film and forms small holes in the film that are difficult to remove when sanding. Use insulation for milled areas - less filler will be absorbed and the wood fibres in the panel will lift less, making sanding easier.

3. The primer coat should not be very thin and sanding should result in a very smooth surface.

To achieve a mirror sheen, the varnish must be applied to a very smooth surface. To obtain such a surface the primer must be very well polished. A good sanding is obtained by making several passes with the sandpaper on the sanded surface. That's why you need a hard, thicker coat of primer. In general 2-3 coats of good quality primer are applied. There are areas (such as coffins, where gloss is a basic requirement) where up to 4 coats of primer are applied.

Those who obtain very glossy surfaces use polyurethane or even polyester primers because they are hard and have a large body (the solid substance that remains after the solvent evaporates). You don't apply one very thick coat but several thin ones. In thick layers, tensions build up and there is a high risk of them cracking.

Sanding is done at least after the first coat of primer - to remove the raised fibre - and before varnish is applied. Ideally, sanding should be done after each coat of primer. Sanding removes film imperfections (orange peel effect), dust particles trapped on the surface and small craters left due to trapped air. Do not use sandpaper under 400 because small scratches that occur when sanding will be visible through the glossy lacquer layer.

4. The chosen lacquer must be high gloss and of good quality

Good quality varnishes do not dry quickly and spread very well on the primed surface. The drying time is longer precisely to give the varnish time to set so that the surface is perfectly smooth and the light reflection total. Don't force dry gloss varnishes, you won't get a quality product.

High gloss means a gloss level above 90, even 95 gloss. Find here information on the classification of lakes in terms of their degree of gloss. Good quality glossy varnishes also have a very good transparency which enhances that perfect glossy feel.

5. For a deep gloss, the varnish layer should be thicker

In general, the recommendation when finishing is to apply several coats of primer and only one coat of varnish. It is the varnish that gives the surface qualities of the varnish - hardness, resistance to scratching, abrasion, hot or cold liquids, staining and one coat of varnish is enough to give the surface these qualities.

For a gloss finish, however, it is recommended to apply 2 coats of varnish, 30-45 minutes apart, without sanding. This gives a thicker coat of varnish that gives the finish depth. If the varnish also has very good transparency, the effect is very clear and very adamant. The brightness and depth will bring out the natural wood grain very well.

In the case of painted MDF, a layer of clear gloss varnish is applied over the paint to achieve this depth. In addition to this effect, the varnish also provides very good protection. In the case of fine scratches, only the varnish will be removed, not the paint, and the defect will be very inconspicuous.

6. The equipment used must apply uniform quantities and incorporate as little air as possible in the film.

The application of finishing materials is usually done with spray guns. This method introduces air into the film. Sometimes this air doesn't have time to escape and gets trapped in the film giving a rough appearance and ruining the gloss. The less air that is trapped in the film, the better the finish will look.

7. Sometimes, even if all the rules are followed, polishing is still needed

It is not easy to meet all the above conditions. Often, despite the best efforts, the images are not perfectly reflected by the surfaces obtained or small particles remain trapped on the surface. Perfection can still be achieved by polishing, by buffing the surface with very fine abrasive pastes.

Details on how to polish can be found at here. What I want to stress is that you need some experience to get very good quality surfaces. It's not impossible to do and you don't need special courses to do it. But the refinement comes over time, after you have done the operation a few times and have caught the little secrets.

A lot can be said about the glossy finish and the risk of getting lost in the details is high. That's why I prefer to stop here. I would be happy if you would leave any unanswered questions or any uncertainties in the space provided. I promise to answer each of you.