Position a hand filled with drywall screws in the tank and shake them about vigorously for many minutes. These will dislodge any unfastened particles of rust. In the situation of Fiberglass and plastic tanks, this could rough up the tank interior, improving upon adhesion. This can be my particular advice https://caswellengineering23455.ageeksblog.com/36291843/the-cibse-approved-cpd-diaries