Hi all. One guy brought to me his guitar for some work and maintenance, and it's covered in unbelievable amount of that gunk made of grease, sweat, dust, cigarette smoke and whatewer it could be else. Never seen that much. So, everywhere on the net they say to use naphtha to remove this gunk, but in my country I actually never heard of a "naphta" and I can't find it. What's the best and safest substitute for it? Some sources says it's a petroleum spirit between benzene and kerosene, some says it's actually a white spirit, another ones - that it's a regular lighter fluid. Which of these I can safely use instead a naphtha to clean off this thick layer of grease without worrying about ruining the guitar finish? All that I've tried up until now (various clean&polish stuff) seems to be little help or building up cloudy stuff on the finish. Thanks in advance.