How to finish wood so that the surface is coloured, transparent and smooth "glass"

For many wood enthusiasts and even very good carpenters, wood finishing is not their forte. That's why I'm choosing to give more elaborate explanations, hoping to be of use to those keen to know more about staining and protecting the surface of wood. Here I will present the solution to obtain a smooth "glass" surface, through which the design of the wood can be seen very well. I would like to make it clear from the outset that I will not be referring to epoxy resin coating, which you will learn about here and here. It will be about finishing the wood used on the inside because, as I explained, on the outside, semi-transparent or opaque products are required so that the finish is durable.

To achieve that perfect surface, attention must be paid to several aspects

There is no miracle material that you can apply to wood and get the glass surface. The result is achieved by using quality materials, suitable for the final purpose, respecting all the working steps, sanding the surface well and working in as clean and dust-free an environment as possible. Each of these elements has its own importance and one is not possible without the other.

Such a finish is not easy to do. In order to achieve a 'glass' coloured wood finish, several aspects need to be taken into account:

• Colouring. A material should be used that does not cover the wood design but highlights it.
• Polishing. It is important both to sand the wood to get an even colour and to sand between layers to get that glass-like smoothness. If you want the surface to be glossy too, you will most certainly need to polish it at the end.
• Materials. Primer, varnish and number of coats applied. The transparency, depth and smoothness of the film depend on them.
• Application. How to apply materials
• Working environment.

Being a difficult finish it is much easier to achieve under factory conditions. There, the machinery used to apply the materials is professional and efficient, and the working space is suitable for achieving a smooth surface and a good, glass-like gloss. If we're talking about a DIY project, it's harder to achieve a perfect surface in terms of transparency. DIY store lacquers don't usually have the necessary clarity, and brush or trowel application is not the best solution to get a film with depth. But it's not impossible if you choose your products carefully and give extra care and time to the application.

Pay attention to the materials used

Colouring is made with transparent baths. What does this mean? They are baths made with dissolving dyes, not pigments. With dyes that dissolve in water or solvent, clear solutions are obtained. They absorb into the wood and stain it without covering it. The advantage of such solutions is that they bring out the design of the wood. The disadvantage, for those who like a uniform look, is that they also reveal imperfections in the surface and do not hide any defects in the wood. In the end the wood, stained will be perfectly visible, but the appearance will not be uniform except in rare cases, when the wood does not absorb very differently and has uniform design. Woods with pigments in their composition are not clear because the pigments do not dissolve, forming dispersions. Applied to the surface, they will partially cover the wood. The stain will be more uniform but semi-transparent.

Grundul is necessary to get that smooth surface on which the lake is finally laid. As in mirror gloss finishFor the varnish to spread well and achieve a glass-smooth surface, you need a material that sands well. Sanding removes any imperfections in the film - air or dust trapped in the film, orange peel appearance - resulting in a straight and smooth surface. The primer should also have good transparency. This means a highly filtered product and no fillers, which makes it more expensive than other primers.

Lake should be transparent, hard and as glossy as possible. Matt varnishes contain wetting agents that spoil transparency and clarity. The hardness of the varnish also contributes to the glass-like appearance of the varnish. In the past, polyester varnishes were used to achieve these glassy surfaces. They were very hard, transparent and full-bodied so the finish had depth. For comparison, if you used nitrocellulose varnish you needed 10-15 coats, compared to 3-4 coats for the whole polyester system. Now the most beautiful 'glass' surfaces are achieved with polyurethane varnishes.

Sanding is important at all stages: also in staining and priming and varnishing.

Coloring, wood sanding is important because it ensures the uniform absorption of the berry into the wood. Sand with wood-specific sandpaper with a final grain of 150, 180, even 220 if the wood is soft and smooth. At lower grits, the absorption of the wood stain into the wood will be greater and more difficult to control, and when the paper is finer, the fine dust formed prevents the colour from entering the wood, resulting in stains and very light staining. After drying the bath, the wood does not sand, even if the wood grain has risen. Sanding to remove the raised grain is done after the first coat of primer so as not to spoil the appearance of the stained wood.

The primer is sanded to ensure adhesion between coats and to achieve that smooth surface mentioned above. Sanding is done with finer paper, min.400 grit for glossy finishes. Grits of 280 or 320, often used for sanding primer, are not recommended for very transparent and glossy varnishes as there is a risk that the fine curls created by sanding will be visible. Information on how to choose sanding materials depending on what you are sanding, find here. Apply at least 2 coats of primer, but up to 4 coats can be applied to ensure film depth. The smoothest surface is obtained by sanding the primer after each coat. After sanding, the surface should be roughened because the remaining dust can spoil the transparency and adhesion between coats.

If the varnish film has an orange peel appearance, it should be buffed to achieve a smooth, glass-like appearance. The same applies if the medium has a lot of dust that gets caught in the film. For good results polishing is done in stages, starting with fine sandpaper of 600, 800 and going up to abrasive pastes of 1000, 2000, 3000 and even finer grits.

About polishing find information here.

Working steps, step by step

Because the most beautiful surfaces are achieved with polyurethane varnishes, the steps below are for polyurethanes. But other varnishes can also be used - acrylic, water-based, nitrocellulose - the sequence of operations being the same. Here are the steps:

1. Wood sanding, air blasting and cloth wiping.
2. Application of the bath. A more even surface can be achieved by applying excess bath and then wiping with a clean cotton cloth. This method is recommended for water-based stains and is also a way of highlighting the natural grain of the wood. There are techniques for smoothing by spraying solvent-based stains, but unfortunately they hide the wood grain.
3. Drying the berry. Solvent-based baths dry faster than water-based baths. The latter are recommended to dry for at least 8 hours. It is best to follow the times recommended by the bath manufacturer. When it dries, the appearance of the varnished wood becomes dull, slightly greyish, without the brightness it has when wet.
4. Primer application. Apply min. 2 coats of primer, with drying and sanding between coats. The quantity applied should be approx. 100-120 g/m². If the resulting surface had many defects and it was necessary to vigorously sand the primer coat to remove them, removing a large part of it, apply at least one more coat to achieve the required depth. Drying time is as recommended by the manufacturer. Polyurethane products generally need catalyzing. The type of catalyst and the proportions are determined by the manufacturer. Changing the catalyst may lead to variations in drying time and hardness of the primer. Use the thinner recommended by the manufacturer. Use of a fast thinner to speed up drying may lead to film gassing. To remove the rough appearance the surface must be sanded very well.
5. Varnish application. Lacquers that give gloss and transparency are more sensitive and should be applied exactly as the manufacturer says, without changing the catalyst, thinner or mixing ratios. These changes can lead to changes in gloss or transparency. In general, apply a single coat of varnish at a weight of approx. 120-140 g/m². For more depth it may be necessary to apply an additional coat of varnish. Optimally, the 2 coats should be applied at 1-2 hour intervals to avoid sanding between coats. If more than 6-8 hours pass, the varnish should be allowed to dry completely and then sanded. After that the next coat is applied. Gloss varnishes need a longer drying time. The manufacturer's recommendation is the one to follow in this case too.
6. Polishing the lake. It is done only after its total drying. For good results it should be done in stages. But polishing is not always necessary. If there is not much dust in the working environment, the exhaustion is good and there is a special space where the varnish is applied, the result obtained after drying can be satisfactory. Especially if the lacquer does not have that very high, mirror-like gloss.

I hope you find the above information interesting. If you are used to getting such smooth, hard and deep glass-like surfaces, please share your experience. Advice is always welcome. And if you have any questions or queries, leave them below in the dedicated space. I'm sure I'll get back to you.