[punyXpress]

Tank off.
Pint of diesel in.
Handful of old nuts in.
Shake tank like mad until tired.
Repeat shaking.
Drain filthy diesel out.
Rinse with more diesel until clear.
THEN apply sealant.
Tank back on. Fill with PETROL.
Ride until the cows come home.
ALSO : check filler drain isn't rusted through - there's a few threads on how to fix.