After several months of flying my UCD, the stopper on my tank came loose as well. I noticed fuel dripping out the bottom of the plane as I was fueling it up.
K2R spot remover is the spray if you can find it. It has been banned in some states by state level EPAs. I can't get it in Indiana but I have ordered it online. It is the best for this application. Spray it on and ley dry an hour or so. do this a couple times and your good to go. Monokote will stick to treated areas where other oil drying methods require some kind of sealer to get monokote to stick.
Good Luck,,,basmntdweller