This is my process, and it takes a bit more effort and time, but
no gnats. When used in an assembly line manner, it goes
faster than one would expect.
For this year's seeds I put used potting soil from pots that have been outside all season, along with some new potting soil, so the possibility of having gnats and their eggs was risky, I'm guessing. I needed to use the old stuff to have enough soil, and avoid having to make a trip out to buy more. I don't usually use old soil, but it
worked out okay.
Filled the pots with the soil, then lined up several dishpans, and
placed the pots in the dishpans. Boiled water, rapid boil, and
poured into the pots. Poured enough water in the pots to also
fill the dishpans about 1/3 full. Everything very hot and steamy
from top to bottom. The fragrance of cooked soil filled the air.
Let the whole thing cool down enough to plant seeds, put plastic
wrap on top of the pots secured with a rubber band, hang on the label,
and then time to go back to the boiling water process again.
Plants are upstairs in front of double doors, and it is quite warm
there, along with dry winter air. Heat rises ya know; wish it was that
warm downstairs. The plants (dip or tet) germinate within a week
after being in the fridge dry packed. I like dry pack due to less
moldy issues, and it keeps the seeds from germinating so I can
decide when to plant them, not the seed's decision.
I used to have so many gnats that I vacuumed them up to
knock down the numbers, but had to resume vacuuming
when new egg hatching occurred.
Works for me. I haven't seen a single gnat, seeds planted early October,
and I also do not have any potted house plants which might harbor gnats.
