I do the same thing. I throw the bag into the washing machine, gentle cycle, cold water with a little detergent. The whole thing comes out looking like new- inside and outside.
I use my XL as an overflow carry-on bag that I do check on occasion. The last trip, I was forced to check it. There were tons of grease/black marks on it. All but one came out by washing it in the washing machine.
I use my XL the same way -- a carry on that I can check if I need to. I love how there's a little hole through one of the leather pieces so you can lock it.
i only have the black ones so i dont have a prob with dirt so im no help lol but its starting to wear out on the corners i wonder if i can fix it some how???