Rub the rust down and apply this first before your underseal
http://www.ebay.co.uk/itm/Granville-...AAAOSw3ydVthem
and inside the sill squirt some of this inside <make sure you get the extension nozzle
http://www.ebay.co.uk/itm/2-x-DINITR...sAAOSwtnpXoFa7