In Fahrenheit 451 Montag says that "you never wash it off completely" referring to the kerosene. What could this mean symbolically?
When Montag first meets Clarisse, he is coming home from work, and she accurately guesses that he is a fireman. That is when he explains to her that the smell never really comes off. The literal meaning of that is that because he uses kerosene at his job, it's hard to wash off, and so he always smells a little bit like it. It's kind-of like a car mechanic who always has car grease under his nails. It's just one of those things that happens in that profession.
To look at a deeper, or symbolic meaning, you could connect it to how later in the book, Montag feels horribly guilty for the books that he has burned, and the people that have died as a result; that knowledge and guilt "never washes off." He has to carry the burden of what he has done as a fireman with him for the rest of his life, like a weight on his shoulders. He can't get rid of it, just like he can't get rid of the kerosene smell from his job. By the end of the novel you will see Montag do a complete turnaround--at the beginning he loved his job and kerosene was "like a perfume" to him. By the end, he will have done very drastic things in the name of rebellion and independence, and that is something that he has to live with. He can't shake off his old life cleanly; the repercussions will stay with him. I hope that those thoughts helped; good luck!