I bought a flame gun but could not use it as the gas used gave me asthma. I gave it to my daughter and she reported that the weeds soon grew back after she'd used it. I have a similiar problem with my raised gravel bed and I really think it needs doing again unfortunately, or at the very least another layer of gravel on it. I hand weed mine and then when I get desperate I use "round up". Good luck with yours.