Re: Sunning stains out, should be a sticky.
The wetter you get the stained fabric before sunning, the better.
Stains will come out faster if they're exposed to direct sunlight during the hottest time of day. Stains come out from sunning indoors, too, it just takes longer in my experience.
Adding lemon juice to the stained area helps a lot. You should wash the diapers afterwards, though.
Here are some videos:
In my experience sunning stains is very effective for biological stains: vomit, poop, spit-up, etc. It hasn't been quite as effective for marker stains, red juice stains, etc. Has anyone else experienced this or am I imagining things? hehe