Pre wetting concrete always works for me. I pre-wet all surrounding surfaces when using F-13 & post rinse asap. I also start with a week solution of F13 and increase as needed up to no more than the max recommended keeping an eye on how the paint is affected.
Most of my customers are happy with how the gutters look after washing the house anyway & don’t want to pay for gutter cleaning. Even at $1/lf for one story & $2/lf for 2 story its not a win for me really so I don’t market it. Also, seems like customers who can afford to pay for gutter cleaning probably have wood facia & soffit and if the gutters are that oxidized, it’s probably time to paint the exterior trim anyway.
Sometimes I will do the 1st story gutters at the front entry if they look bad as that is what most people see. Light striping on 2nd story gutters are not even noticed by most people unless it is pointed out to them.
I have found that it will remove bled oxidized paint on vinyl below painted shutters but I always check for oxidizing on the siding in that area & explain to the customer that the finish on the siding may look different than adjacent areas. Most say go ahead because they would rather have the paint stains from the oxidized shutters removed because it make the the front of their home look awful.
In general F13 seems to have mixed results for me. Sometimes it works great & sometimes not. Maybe has to do with age of gutters, manufacturer, if they have been painted post installation? Also, on an older home I will always do a test area in an inconspicuous place. The older the paint, the more likely F13 will remove it along with oxidation. Plus, the stuff is wicked, so you have to use more PPE, which is time & money.
Overall, it’s a good product & i’ll always keep some on my truck.
More info than you may have wanted? but thats my take on F13