Every drywaller I've ever watched adds water to the bucket of drywall mud before they mix it up,goes on easier and smoother from what I've seen.
I would think it does the same for latex paint.
If it doesnt cover,they could just add another coat of paint.
This post touches on something not many think about. In the case of mud, it's easier to put out a product that needs adjustment to suit the conditions, e.g., outside weather and indoor temps. Humidity is a big factor. Yo can't make it thicker.
Another point is that a manufacturer will never err on the side of too thin. When they do, they get complaints up the ***. It's happened to me, a painter now for 50 years (I do other work but I painted first in the union and then on my own with a crew for 10 years. I became a general but I never totally left painting. I paint something every week).
Paint is made from solids and liquids. Within the liquid part there is the binder resin and the part that is designed to evaporate (reducers). It should be spread out to spec always listed on the spec sheet which you don't commonly get with a purchase. Specs are available at a pro store or online for almost any brand.
If you thin the binder and reducer too much, you violate the specs but it takes more than 10% to ruin a gallon of paint. I prefer 5% by volume to start. Using a flowing agent in no more than using a touch of antifreeze which is used in a pinch on hot days to retard the paint some.
The trick is always to adjust paint so that your additives evaporate according to the specs leaving the binder behind to create a film that is not compromised. Everyone has their own ideas and no one knows really what they are doing.
Here's the kicker and I have it from the horse's mouth: paints sold in CA have to be compliant AFA VOC's go. The paint is right on the margin and if you thin it that makes it non-compliant.
Yes, it works better that way and the manufacturer's know that but they can't say it. You say water is inert and can't add to the volatility? Well it does in a complex chemical way that we don't even need to know about. What goes on at your job is of no concern to me and I hope vice versa.
Paint that comes to the end user as a "brush, roll or spray" product cannot do all 3 equally. Spray equipment needs a certain consistency to function optimally and the velocity of the spray helps with rapid evaporation of the reducing agents giving you the required mil specs called for with each coat.
This is a very complicated issue of working with coatings under varying conditions and is not a subject for people to just toss out observations and unfounded opinion. If you haven't put up 1000's of gallons and been to an actual paint manufacturing company to learn about this, you are not qualified to express what you think is fact.
To the OP: I would have left too but it's entirely your fault. You do sound like you opened up the material ostensibly to box it up for color constancy and automatically added water like adding salt to food w/o first tasting it. You could have demonstrated how the material behaved right out of the can to your customer. You could have used a painter's secret and over brushed it until it looked ropey and then told the customer that condition can be dealt with by adding a reducing agent. If the can mentioned water, there ya go.
When I paint a slab door side with a roller and a brush to tip it off, I allow 90 seconds to begin and get off that side. Paint always lays down as if it were sprayed. You guys know what happens when you go back to "touch up that little spot." Now you have a big spot.
