A/C units sweating in the attic

Sweating or normal condensate is a byproduct of air conditioning. Dripping in the pan and will continue to do so until the humidity is 20-30 points lower then it will slow but continue to drip (that’s why there is a pan to catch the water or condensate) Once the water is caught it can be pumped or piped out of the home. If you have sweating on the outside of the unit you are lucky your pan catches it. Insulating that metal surface can reduce or even eliminate sheet metal sweating when sealed with insulation.

So sealing is a start but insulating the ductwork Will be needed because as long as there is cold metal in a high humid area it will sweat.

If you have a glass of ice water is it sweating? Your metal parts are the same.

Air contains water. Warm air can contain more water than cool.

True fact. Air wants to contain moisture and water will directly evaporate into the air wherever possible. If you ever breathe air that doesn't contain water, you know it immediately because it steals moisture out of your mouth, throat and lungs. Aircraft pressurization is somewhat like that.

How much water can air possibly hold? That depends a great deal on the temperature. Warm air can hold much more water than cool air. And that is why we express humidity in terms of "Relative Humidity" instead of an absolute unit like grams per litre. RH is the percentage of water in the air compared to the maximum at that temperature.

When you chill air, the water falls out.

That is why an ice cold glass will have condensation. It is not leaking through the glass. The air that is very close to the glass is being chilled by the glass; when its temperature falls, it cannot hold as much water. The water has to come out. So the water grabs onto the nearest surface, and there you are.

As you can guess, an air conditioning system works by passing warm air through a chiller. This transfers heat (makes the air cold). What happens when you make air cold? Bingo. You reduce its capacity to hold water. The water must fall out of it.

That is why ever air conditioning unit, every single one, that has ever been made, needs to contend with condensation on its evaporator coils. In massive quantity. Like, a 1/2" drain line is not excessive.

That drain line has to go somewhere, noting that the condensate can be a bit corrosive, since it is not mineralized like normal tap water is. It wants to be mineralized and will suck minerals out of piping. It's not grey water and can simply be overboarded.

Aside from the evaporator, other parts of the machine may also be cooler than ambient. Those too will tend to collect condensation, especially if they are poorly insulated or the insulation has been defeated by a careless maintainer.