I agree with Clive's suggestions.
I have just printed two test strips to see if the stains would be formed. On purposely, I poured and started coating from the middle. I must say, although they are quite weak (probably due to acidity), they are visible. and that is not good
Originally Posted by cliveh
If you pour SN to the edge of your coating area and later mask that part, it might also work.
But the simplest way seems to be dipping the brush into SN as clive suggested. I would do some tests tomorrow.