A customer asks for gloss. So you sand finer. 320, then 400, then 600, chasing the shine up the grit numbers. The surface gets smoother and the shine doesn't come. You buy a higher-grit paper and try again. Still flat.
You were never going to get there that way. Gloss does not live at the scale sandpaper works at. It lives about a thousand times finer than your finest grit, and no amount of paper closes that gap. The shine on a glossy top is not sanded into the wood. It is borrowed from the finish, from the way a liquid coat flows out nearly dead flat before it sets. Sandpaper controls how cleanly that finish can lay down. It does not make the shine.
That is the first of three things customers ask for and routinely get wrong, and once you can name all three you can answer the client across the bench instead of guessing.
Start with gloss, because it is the one everyone fights. A gloss number comes from exactly two things. One is the finish itself, the resin, which sets a ceiling on how much shine is even available. The other is how smooth the surface is, which sets how much of that ceiling you actually reach. And the smoothness that matters is brutally fine. Half the shine is gone by the time the surface is rough at a scale of a few tens of nanometers. Sandpaper works in microns, roughly a thousand times coarser than that. So you cannot sand a shine into bare wood. What you can do is sand so the finish levels cleanly on top, and that is worth doing, but it is a different job from making the gloss. The gloss comes from the coat flattening itself out as it flows. Pick a resin with a high ceiling, let it level, and the shine is there. Sand all night and it won't be.
Now depth, which is two different things wearing one word. A customer says they want depth and they could mean either of two things that cost wildly different amounts of work. One is the look-into-it clarity you get from a clear, well-behaved finish even in a thin coat. A couple of coats of shellac can look bottomless, and that depth is optical, it is the finish being clear, not thick. The other is literal build, a genuinely thick transparent layer, the thirty-coat piano top where the depth is real physical glass you put down one coat at a time. To the customer they look the same in a showroom. To you they are a weekend versus a month. So before you quote coats, pin down which depth they are asking for. Ask whether they want it to look deep or to be deep. The answer changes the price.
The third one is the request you have to turn down, and it is the one the reader will use that week. A customer wants a deep, rich black that is also dead matte, no glare. You cannot give them both at the extremes, and not because you lack the skill. Matte is made by deliberately roughening the surface so it scatters light in all directions. That same scatter is what raises haze, and haze is what turns a black milky and grey. Rich black and low glare pull against each other in the physics. Push the matte all the way and the black goes flat and chalky. The honest answer is not a flat finish, it is a high-clarity satin. It reads deep, it reads black, and it keeps the glare down without the milkiness. Tell the customer that before you spray, not after.
So three corrections, three things to say out loud to a client. You can't sand in a shine, so pick the finish for the gloss and sand so it levels. Depth is two things, so ask whether they want it to look deep or be deep. And a dead-matte rich black isn't physically available, so show them a satin that gives most of what they're after. The resin sets the ceiling, the smoothness sets how much of it you reach, and sheen is something you choose on purpose. It is not something you grind your way to with finer paper.