Paint uniformity drives appearance

There is no question that whatever surface is being painted there is 'the correct' paint thickness which gives the best look (appearance) and durability of that surface. For example, imagine painting at home a wooden doorwith gloss paint. How many times have you seen after the paint has dried sags and runs (too much paint) at various locations or areas where you can still vaguely see through the 'old' colour (not enough paint)? Personally many times and it can be frustrating because more time, effort and paint is needed to rework this door applying the correct thickness everywhere making it look its best.