I do pretty much the same. I explain to the customer at the time of the estimate about the difficulty with making gutters look good. The customer needs to be aware of the fact that when you apply a stronger cleaner which is necessary to clean the gutters, We run the risk of taking the paint down to the aluminum.
I've battled the gutter problem for almost 30 years now and know how the gutter finish is going to respond to the cleaners. Since most gutters(at least the ones around my area) require an Alkaline cleaner, They will remove a layer of oxidation. Question is, Has this been done before and if so, How many times??? I explain to the customer, Each time this is done, A microscopic layer of paint is removed and since the layer of paint is fairly thin to begin with, The finish can only handle a few times of the aggressive cleaners.
At least if the customer is aware of the risk involved, They can(hopefully) make an educated decision as to the risk involved. I access the condition of the gutters upon writing an estimate and try to determine the durability of the gutter finish and I've had my share of customers whom don't seem to believe me when I try to explain it to them so, I go to an area which is hard to see(if possible) and perform a test. I've had gutters which went straight down to bare aluminum and stop to show the customer. Then they understand and suffice to the fact, The only way to make them look good at that point is to have them lightly cleaned and painted by a professional painter or simply replace them.
I try to explain to the customer of the fact that the finish on gutters is not the same kind of paint as most other painted items are. When the gutters come to the home, They are a big flat roll of painted aluminum. In order for the paint to survive running through the "on-site" bending brake and rollers, The paint needs to be a "soft paint" otherwise it would crack and start to peel. Therefore, It cannot handle too much aggressive chemicals and being a soft paint, That's why it absorbs stains and doesn't want to release them.
Educating is the key here!!! If you can educate the customer in a way the competition doesn't, You will most likely land the job as the customer will(hopefully) realize you know more than the others and truly care about the end result rather than taking their money and running!!!
All the best,
Mike