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"Pasty Butt" is not an issue related to your bedding choice, though it *can* be an indication of birds that are too hot, not hydrating, not properly digesting their feed, etc. Some folks have luck with adding ACV to their water, some like to feed soaked or even fermented feed - check your brooder temp (consider the chicks' behavior rather than a number on the thermometer), make sure everyone is drinking enough (this is where wet feed comes in handy) and keep up with cleaning them up as you notice them pasting. Applying a little petroleum jelly beneath the vent can help prevent further sticking.
This is only my 2nd time raising chicks in a brooder but both times I've had issues with pasty butt on at least one of the chicks.
Both times after cleaning their butt, I added a chunk of garden dirt to the brooder for them to pick at and no more pasty butt issues.
Plus they seem to love the dirt, whenever I check on them they are picking at the dirt.