I had a conversation with someone who, after converting some kitchen furniture, had a problem with cracking paint applied to the wood and didn't know what happened. Convinced that the paint was fine, he thought he might have done something wrong. He described the technology to me point by point and that's when I understood how you can end up with a poor result by doing more than necessary out of a desire to be as close to perfection as possible. In short, transformation involves 3 steps: cleaning down to the wood, hand sanding of wood and repainting. The sanding of the wood was done very carefully, in steps, starting at 100 grain and going up to 500. The paint was then applied, in thin coats, waiting for the necessary time to dry and yet, after drying, the paint began to crack. What happened? Where is the fault?
Hand sanding of wood
Sanding the wood before applying the wood stain and varnish is necessary to obtain a uniform stain, to ensure that the film formed has good adhesion and that the wood has a pleasant, non-scratchy feel.
Good adhesion means good absorption of varnish/paint into the wood. Wood naturally absorbs through fibre channels (visible in some species as pores), exactly where water circulated when it was a tree. The area where it absorbs best is where the fibres are sectioned. This is where the so-called fibre heads appear, where the liquid enters easily.
Sanding has this ability to open the fibre ends. But, paradoxically, it can also close them, limiting absorption into the wood. How do we know if we are opening or closing the fibre? By choosing the right sanding paper or abrasive sponge.
Wood sanding paper grain
I won't go into details about the sandpaper. I did it on another occasion and you can find detailshere. I just want to point out that the numbers written on this paper have a meaning and are very important. Choosing the paper according to them will give you a quality sanding. The grit is represented by numbers from 40, 60, 80 up to 400, 500, 600. The higher the number, the finer the paper and the less "scratch". Very abrasive, with low numbers, is used for wood and fine for sanding between coats of primer, varnish or paint.
The fibre ends open up if sanding paper is used up to 180, maximum 220 and this is the case for the more loose and scaly species of wood with soft fibre such as poplar. In hard species such as oak treewe can stop at 150 without any problems. Abrasive papers finer than that no longer have the strength to tear the fiber open. On the contrary, they will push the already open fibres into the wood, closing the absorption channels. The fine dust resulting from sanding with fine paper also helps to close the channels, which will only "grind" the wood fibre, without opening it.
This is what happened in the case described. When the wood reached a grain size of 500, it was totally "closed" and could not absorb any of the paint applied. The film formed was actually a flake of paint on top of the wood, trapped only from place to place. With no adhesion to the substrate, this very thin suspended layer immediately "breaks" at the slightest movement or touch.
Why progressive grinding is done
From the above, the question arises: why then is there a need for progressive, increasingly fine grinding? Have the fibres not been opened by 40 or 60? Why not stop there? Because the wood would absorb a lot and uncontrollably. In the case of wood stains would automatically appear, and in the case of paint or varnish, the large amount absorbed deep into the wood means losses. It is therefore recommended that the first coat of finish (after staining, if applicable) should be grund or the varnish/paint to be more diluted. This will allow it to penetrate the surface area of the wood, to stick well to the grain and the resulting film to adhere.
By sanding the wood progressively, the first grains break the grain and the next ones close it a little so that the absorption is more controlled. This also limits and reduces the size of the broken fibres, which will be loaded with varnish and rise, giving a rough appearance after varnishing. If we stopped at 40 grit, these fibre heads would be very large. Loading them with varnish would result in real wood spikes that would be very difficult to smooth through. sanding between layers. You would need to insist, maybe even use coarser paper, below the recommended (240 and up), which will almost remove that first coat of varnish. Obviously, that means waste and "work in vain".
In conclusion ...
Don't try to get the wood as smooth as possible because you will reduce the adhesion of the varnishes or paints used. Don't leave the sanded wood with only very small grains (40, 60) because you will get stained and dark wood or you will unnecessarily consume primer/lacquer/paint and sandpaper trying to get the finest and most pleasant to the touch surfaces.
I hope I have managed to explain in a way that everyone can understand what happened in this situation. If there are any questions or comments, please leave them below. And if you know someone who could use this information, please share the article with them.
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Hello, from what I've read, I've mostly taken in what NOT to do. Specifically, what are the exact steps? E.g.: sanding 40-60 grit, paint coat 1, sand xxx, etc. I want to re-paint some wooden doors. Thank you.
Hello!
From your question I don't understand if you want to apply the paint over the existing one or you want to clean the old coat down to the wood, then apply another finish. I'm trying to address, briefly, both situations:
1. Over existing paint. You must degrease the surface with water and detergent (wash it), then wipe well and let it dry. Lightly sand with 280 or 320 grit sandpaper, dust and apply new paint. Sanding is done so that the new coat of paint adheres to the old one. It is good to test beforehand if the paints are compatible. Check on a more hidden area. If it spreads well, without forming cracks or islands of paint, they are compatible. To avoid problems, use water-based paint. It is compatible with most types of varnishes and paints.
2. On wood. Remove the old layer by sanding with a sanding machine, pickling or by heating. Below is a link about removing old paint layers. When the wood is clean wipe gently with a cloth with thinner and let it dry. Then sand with 150 grit sandpaper (if the wood is hard - oak, ash) or 180-220 for softer wood (lime, poplar, fir, spruce). Dust and start applying the primer. After drying, sand with 280 or 320 grit sandpaper and apply the paint.Sanding between coats will help to obtain a smooth, smooth surface.
I have also attached a link with tips for finishing doors.
All the best!
https://revistadinlemn.ro/2017/09/15/cum-se-curata-stratul-vechi-de-vopsea/
https://revistadinlemn.ro/2017/08/16/sfaturi-pentru-finisarea-usilor/
Hello. I would need some advice from you. We have a cottage in the country with old wooden doors and have decided to refurbish them. We blew off the old layers of paint with a hot air blower, and then sanded with 60 grit sandpaper, then putty knife and sanded again. Please tell me what to do next. I wouldn't want to ruin all that work. The doors I think I'll paint them super gloss white. Thank you!
Hello!
Indeed, it would be a shame to work. 🙂
My opinion is that you should sand the entire surface with a finer sandpaper (150 or 180, even finer) to have a pleasant surface to touch. Then apply 2-3 coats of paint, drying between coats.Drying time is depending on the paint used. If possible, sand the first coat after drying with sandpaper of min 400 if using gloss paint. You will get a very smooth and pleasant to the touch surface.
I think the very glossy paint is picky and hard to maintain. You can see all the hand marks and surface imperfections on it. I would use a semi-gloss or satin paint. It has a nice sheen and is much easier to maintain.
All the best!
Hello. Thank you for your reply. You told me to apply 2, 3 coats of paint with drying between coats. If I apply a coat of white primer, sand and then apply white satin paint is that ok? Or is there a secret to applying multiple coats of paint? Thank you very much.
It is very good with primer and paint. I was thinking you are a beginner and trying to find a simpler version. 🙂 As you say - a coat of primer, sanding and then paint - is the correct technology.
Good luck!
Hello, we would like to clad the steps of our home (which are now tiled) with laminated beech wood steps.Please let me know what the work steps are and what to give them. We would like the oak colour. Thank you in advance for your reply.
Good evening!
Oak steps can be finished separately and fitted afterwards or fitted and then finished. To minimise disruption it is best to have the steps finished elsewhere.
The steps must first be sanded very well because beech wood is difficult and stains easily. The last sanding should be done with 180 or 220 grit. Sand and apply the oak wood stain. I recommend solvent-based stain because the risk of staining is lower. You can find the stains in DIY stores. After drying, apply 2-3 coats of parquet varnish. You can use water-based or solvent-based varnish, applied with a brush or with stiff sponge pads. If the varnish is solvent-based, the trowel must have the solvent-resistant sponge. After the first coat of varnish has dried, sand with 320 grit or fine abrasive sponge to remove the raised wood grain and achieve a smooth, pleasing finish.
After finishing the steps are glued to the concrete staircase. It is best to remove the layer of tiles so as not to raise the step too high. Bonding should be done with moisture-resistant adhesive (D3 or D4).
The same technology is also used if the finishing is done after gluing the steps.
Good luck!
Hello. I have a pine table that has not been varnished or treated in any way. It has been sitting in the kitchen, next to the stove, and is stained with spilled grease.
I would like to know if I can "recondition" it in the sense of if I could do something to remove the stains and possibly lacquer it afterwards.
Does it need to be sanded? pickled? I don't know anything about it, and your answer would be very helpful. Thank you very much.
Hello!
If the surface is heavily coated with hardened oil that almost forms a film, a combined cleaning, sanding + pickling will be needed. Apply a stripper first because hardened oil is hard to remove otherwise. I leave below a link with types of stripper and how to use it. Caustic soda can also be used, but the method is more dangerous. After the oil has softened, remove it with a scraper or wire brush, after which the surface wiping thoroughly with universal thinner. Finally, the wood is sanded well along the grain with 150 or 180 grit sandpaper or a medium abrasive sponge. The wood will become clean and pleasant to the touch and can be varnished or painted.
Good luck!
https://revistadinlemn.ro/2022/09/08/cum-sa-cureti-cu-decapant-straturile-vechi-de-vopsea/
https://revistadinlemn.ro/2016/04/11/cum-trebuie-facuta-slefuirea-in-alb-a-lemnului-pentru-a-obtine-un-produs-de-calitate/