A consequence of properly sanded wood is also less absorption of finishing materials. Rough, unsanded or sanded wood with coarse sandpaper (60, 80) absorbs a lot and unevenly, stains appear when staining, the first coat of primer or varnish applied disappears into the wood and consumption increases. It is therefore recommended that sanding be done in stages, with increasingly fine-grained paper. But how fine can you go with sandpaper? Will it get better and better the finer the paper, or is there a threshold beyond which the results are no longer as good? What problems can arise if this threshold is exceeded? These are the questions I propose to answer below.
What is the maximum grain size of wood sanding paper
The 220 grit should not be exceeded when sanding wood. This grit is also considered very fine, but is used on soft, loose and rough wood in order to achieve a surface that absorbs as evenly as possible. An example of wood that is difficult to sand is poplar. To break up the fibre and adjust the absorbency go for finer paper than is commonly used. For hardwood such as oak tree, the beech or ulmul, 150, maximum 180 is sufficient to obtain a fine surface that evenly absorbs the materials to be applied for protection and aesthetics.
Wood sandingThe grain, especially the solid one, is made with several grains to gradually break the fibre and reach the desired fineness. If we go straight to fine sanding the roughness left over from mechanical processing, the paper will not last and will wear out very quickly. As a result, the sanding will be poorly done and will increase paper consumption and therefore costs.
It is recommended to use at least two different grain sizes, adapted to the hardness of the wood. Hardwood (fag, acacia, oak, cer) is initially sanded with 80 or 100 grit, then 120 or 150 grit paper is used. The softwood (some of resinous, plop, you, arin) grind with 100-120, then go to 150-180, if necessary even to 220. I mention some of the softwoods because, for example, Norwegian pine or larch Siberian hardwoods are not difficult to grind. Spruce, the most widely used resinous wood in Romania, but it is softer and more loose, therefore harder to sand.
What problems can occur if the wood is very finely sanded
It is true that sanding the wood adjusts the absorption and the material consumption will be lower. This is normal, because wood absorbs much more through irregularly cut fibres. To get an idea compare how the absorption of a stained solution looks in a stick broken in the hands or on the knees and one nicely cut with a fine, sharp blade. The blade cut evenly through the wood grain and the dye rises evenly, whereas the breaking was done messily and each fiber absorbs according to the area of absorption created. By sanding all we do is even out all the channels (fibres) through which the colour or varnish enters so that they absorb the same.
What happens when we use very fine paper? Instead of opening up the wood's absorption channels (fibres) evenly, we block them with the fine sanding dust. The holes get smaller and smaller, and some close up completely and no longer absorb anything. This is how light stains appear when we apply the stain or lack of adhesion of the film. Instead of getting well into the wood and sticking to it, the primer/glue just goes in from place to place, where it is still possible. After drying, the film formed will be very sensitive and will peel off at the slightest knock or mechanical shock. If it is a tougher solvent-based varnish, it will break like glass in the place where it came off after peeling. If it is water-based varnish, it is possible that at some point the film will flake off like sunburnt skin.
In the case of oil finishing, very fine sanding leads to reduced absorption, surface, low protection, low resistance of the finish over time. Refinishing will have to be done faster and more often, water resistance will be lower. One of the advantages of oil is that it gets into the wood and protects it from the inside. By very fine sanding we prevent it from doing this.
In conclusion, if the wood is sanded with a very fine grain it decreases its absorption power which leads to problems of adhesion of the varnish film formed and low resistance in the case of oil. The light-coloured stains that appear when the wood is sanded are another consequence of very fine sanding.
I hope you find the information helpful. If you have any additions, questions or queries, please leave them in the space below. I will certainly reply.
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I salute you. Even though my experience is almost nil, the passion far outweighs any obstacles. Personally, I have come to the conclusion that a great finish can only be achieved with a counterfibre edging, even if we are talking about oak or fir. I have achieved results beyond expectations with a Stanley 9 1/2, of course with care and patience. And that was before I read about Japanese methods.
Hello,
First of all, I would like to congratulate you for the quality and detail of the information on your website, which denotes a great passion for working with this noble material, which is wood.
Secondly, I would like to ask you for some advice. I have treated an old oak door with antique solution, after having previously sanded it correctly, the last grain being 150. The treatment was done with a very concentrated solution, which became foamy when applied with a brush. Although I left a two-day drying time, the wood did not seem to become completely dry in the end. However I tried applying a coat of varnish over it, and the result was that the varnish did not seem to dry, and the surface of the wood remained greasy and darker still. Now I'm wondering if it might be a good idea to apply some degreasing or stripping to try to restore the surface, before trying a new varnish. Some advice from you would no doubt help me and maybe others like me, why not....
With thanks and esteem.
Hello!
Thanks for your appreciation!
It would be helpful if you could tell me if you used it for the antiquing treatment, to figure out why it didn't dry. Normally these solutions are water based, dilute to get as deep into the wood as possible and dry without any problems. One problem with absorbing the next layers is sodium silicate (glass water) or other silicate based solutions. In these cases the surface becomes very hard and the depth absorption very low.
It is clear that the lake is affected by the substance applied before. It is possible that the substance used inhibits the polymerisation reaction which causes the varnish to dry and therefore the varnish remains tacky. There are many unknowns for me and it is hard to say what happened.
As the lake does not dry out it is obvious that you need to remove it. Use stripping solutions, I doubt you will succeed with thinner alone. Find such solutions in DIY stores. The solution is applied with a brush, allowed to react for a few minutes and then the varnish is removed with a squeegee or blade. If you can't remove it completely, repeat. Finally, wash with universal thinner and sand the surface again with 150 grit paper. After stripping you can resume varnishing. It is a good idea to check beforehand whether the varnish can be applied over the antiques solution used.
All the best!