I harden off when my seedling are about 3 weeks old, 30 minutes the first day, in a shady location, and gradually move to a brighter, sunnier location for longer times for the next 2 or 3 weeks until our last frost date rolls around.
The fan is also being used indoors 10-30 minutes 2 or 3 times a day. I've been rubbing my hand over them a few times a day before starting to use the fan. I have just 41 seedlings going, so one flat and some 'extras' so it is easy enough to do with so few.
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