answer:Hand wash them in a basin. Rinse, hang out to dry on porch or deck chair. I think the residual stuff on the rags would not be good for plumbing. Plus, occasionally, oily rags that are stored can combust. It happened to me once when I was using linseed oil on refinished furniture. Luckily I left the rags in a galvanized bucket, on a screened-in porch, but I see see smoke coming out of them.