This tip from a tropical gardening site using boiling water as a pre sowing compost steriliser.
http://www.cooltropicalplants.com/growing-canna-from-seed.htmlTo save you reading the whole page here is the relevant extract.......
Before planting I like to sterilize the compost. This is to kill off any fungal spores that may wish to attack your babies as they emerge. My preferred method is as follows:
Fill the clean flower pots with a multi-purpose compost (new shop bought stuff, don't use any old rubbish left over from last year!). I like to use boiling water to sterilize the compost surface. Fill a small (pint sized) watering can with a fine rose attached, with boiling water. Water this onto the compost. Using a fine rose prevents an uneven surface. A small watering can prevents you pouring boiling water all over the kitchen floor or onto your legs.
If you do this an hour or so before planting, the seeds will benefit from having a nice warm pot of compost. The compost will also have drained any excess water and be at just the right level of moisture. Pop the seeds in evenly and not too deep.
The writer is discussing canna but I have tried it with a few pots of fine seed (petunia etc) and no sign of damping off at all.