I have worked in many very high end finish shops doing fine furniture and architectural millwork most of my life and have never worked in a shop where we sanded any finer than 220 grit and 90% of the time it is 180. This is using both a wide belt sander and a random orbit sander. We have found that 180 grit is the point where the swirls or pigtails from the RO sander no longer show without using a magnifying glass. For very high end jobs and samples I will alway block sand the final pass with the grain to completely eliminate any possibility of swirls. Some other mistakes a lot of people make when using a RO sander is moving it much too fast, pressing down too hard and not changing the paper often enough.
Trying to make a "rule" when finishing wood is ridiculous because there a LOT of different circumstances you must consider and the final look you are after determines the road you travel down. So for me every different procedure is a tool I can use to match the look I am after as long as I understand how it works, what effect it has on the final finish and the amount of time required to duplicate it.
1. Wood species: Normally you wouldn't sand a soft species (Fir, Pine or Alder) the same as you would sand a hard species (Hard Maple, Oak or Ash) UNLESS that provides the effect you are after. I have done some antique finishes where the designer actually wanted the wood to look blotchy because that looked more authentic in an aged piece of furniture. The exception is when burnishing the wood actually causes finish adhesion issues, this is fairly common with Hard Maple and the woodworking forums are full of posts where shops have had this problem. I don't sand Hard Maple or any of the hard species of wood any finer than 180 grit. The only exception to this would be on end grain on things like chair arms where I know the stain will penetrate deeper and go dark. I will hand sand the end grain only 1 grit finer and soak the end grain with a little clear solvent that is compatible with my stain. With lighter colors sometimes I don't even apply stain to the end grain and adjust the color if needed after sealing.
2. Solvent base of the stain: With a water based stain you can actually sand with finer grits than with solvent based stains because the water in the stain actually opens the pores of the wood up and allows proper adhesion of the finish.
3. Labor hours: In a commercial shop man hours must be kept in check and sanding is the single biggest use of labor. Sanding something all the way through 400 grit is a HUGE waste of time and cuts into the profitablity of the job without providing any addition of quality. In our shop we spray stain on huge jobs of wall paneling in Cherry and other blotch prone woods and have no issues with blotching. We don't use any washcoat, we control blotching by application method only. In my home shop I don't do a lot of sanding at all, I prefer the clean look of cut fibers and do most of my work with a card scaper or hand plane.
4. The belief that sanding alone can control blotching: Blotching is quite simply the stain absorbing into the softer areas of the wood more than the harder areas. Even if you sand everything to 400 grit the softer areas of the wood will still absorb more than the harder parts. This can be used for effect in species like Anigre and Curly Cherry to "pop" the figure of the wood, nobody calls that blotching. They think the figure is beautiful, yet is is actually controlled blotching.
The absorption of the stain can be best controlled by application method. Applying the stain by spraying you can apply just enough to lightly wet the surface evenly, by not allowing the stain to puddle there will be no blotching. Dye stain works best for this because you can apply the stain multiple times without worrying about adhesion issues.
For those who don't have access to spray guns the surface can be pre-wet with the solvent of the stain to saturate the softer structure and limit the amount of stain that will soak in. Another option is to use a wash coat of finish to limit the absorption. One of the biggest issues I see with people using a wash coat is that they think there is a standard mix. Samples must be made using different dilutions of wash coat. I will start with a 1:1 mix and go all the way to a 10:1 mix (solvent:binder) on a single board with a strip of masking tape between each one. When I apply my stain I can see the different amounts of absorption and pick the ratio that gets me closest to my desired color with the least amount of blotching. I also find that wiping on a wash coat is a little better than spraying it on when possible. I see people posting about how their stain is now too light after the wash coat. The reason is too much binder or finish in their wash coat. You still want the stain to penetrate into the wood, just not as much.
Another thing I see in the photos on woodworking forums is most people don't sand enough or improperly. I see mill marks and scratches all the time. The use of a good inspection light or strong cross lighting is essential for proper sanding. The last thing you want is to apply your stain and find huge scratches or mill marks and have to sand everything back to bare wood and start over. The proper use of an inspection light will eliminate re-sanding. I just mark the defects with a pencil and sand them out.
.jpg)
