About once a year my kitchen sink drains very slow. When my husband removes the trap and the first down pipe below the trap they are clogged with a black sticky substance. I don't pour grease of any kind down the sink. Any suggestions as to what to use to prevent this build-up in the first place. I also use Dawn dish soapl
Thanks; Doris

Does your disposal drain through the tailpiece? What you see is fairly typical. Food and other organic debris get in there and bacteria get free lunches. Make sure to flush the trap with very hot water and an occasional cup of bleach sitting in the trap all night will also help. Don't wash your hair in the sink since hair get trapped and food gets caught.

Do as suggested above. Also, let HOT water run for minute after each time you do dishes

Milo