I soak the paper in a tray of gelatin (in the same proportion you use) but I
keep the temperature of at least 40xC for at least one minute then hang up to
"drip dry" I don't squeeze or roll or do anything else to the paper and get
nice even coatings of the size. . The temperature appears to be key in
getting a smooth take of the gelatin to the paper. As the temperature drops
more bubbles and the more likelihood of uneven coatings.
It sounds to me without seeing the "stain" that this is an area of the print
which is not in contact with the water during development. The usual problem
is an air bubble under the paper which is being "still" developed. This
might be the case if there is only one large area of stain not several small
areas.
Larry Shapiro