A thicker coat of polish can look beneficial for a moment. The color looks more vibrant, the brush seems to get more nail plate with each stroke, and one heavy coat seems more time-saving. The problem starts afterwards: the polish slides toward the cuticle line, pools on the side walls, gets smeared easily, or forms small air bubbles in the final result. Thin layers require more patience, but also they are much easier to manipulate.
Polish should not sit like a soft pile of paint on top of the nail plate. It works better when the brush is used to deposit a small amount of polish in a controlled direction. When the brush is overloaded, it is difficult for the beginner to make it look even before it has set. Some part of the nail plate will be shiny, whereas other areas could be flooded or become too thick. The use of a thinner coat provides a greater time frame for the polish to move in a controlled manner without spilling into the adjacent tissues.
Just as important as the amount of polish you use is how you press the brush. If you press too hard, the brush hairs fan out and move the polish too far in some places while leaving too much in others. If you touch the brush too lightly, the polish does not spread properly and you can lose control of its flow. The proper approach is to use less force and keep the brush fanned so it covers the nail plate without spilling over to the cuticle lines. You should not be aiming to fill the plate on the first coat, but to lay down a controlled first coat so you can add to that with a second coat.
See these two practice tips side by side: On one side, use a heavier color coat and observe its behavior after a few seconds. On the next nail plate, remove some polish by pressing on side of the brush against the lip of the bottle, then place a small amount of polish close to but not at the cuticle line, and brush it slowly into three separate strokes on the nail plate. This thinner coat may be slightly translucent, but that is to be expected. Pay attention to a difference in the surface texture, how much easier it is to clean the side walls, and how much better you are able to cap the free edge without a thick, bulky edge.
Use the second layer of polish to build coverage but not to correct all errors made in the first layer. If a layer of polish is too thick, or there are air bubbles or polish sitting on the surrounding tissues, the more polish you apply over them, the harder it will be to make corrections. It is best to examine the polish between layers and see if there are any problems. Look at it in different directions, looking at the cuticle line, the side walls, and in the center of the polish on the nail plate for any streaks or uneven color. If you need to clean up a mistake, it is much better to do it now than to add another layer of polish that will cover over the issue.
Another way that thin coats are cleaner is they work with top coat to dry the nails better. When the polish below is applied too thickly, then the top coat might slide over it as if the coat is even, but underneath you can find polish that has not been completely dried out by the top coat and can be dented or smeared. It may be polished initially but lose its finish over time. When using conventional nail polish, the thin coats dry in a more even pattern. When using gel polish, applying a thin layer of polish is important because too much gel product may not dry properly and can leave an oily film near the cuticle or side wall lines.
Cleaner nail polish is more often achieved by applying less polish than you think. If the nail is being applied in a thin layer and the polish stays where you want it, and the polish is not dragging along the brush, and you will build a second coat that will improve the coverage, then you might want to keep the coat thin. Before you add more polish to the nail plate, stop to see if what is already on the brush would be better used in a thin coat.