Roy,
As my cobwebby brain remembers, for conductive materials such as
aluminum through gold, the rough rule of thumb is that at 10 megacycles
the skin depth is .01MM (01 is 10 backwards, only reason I remember)
So 1mm is 0.0394" therefore a skin depth of 0.1mm is 0.00394", call it
4/1000 of an inch for round numbers... So, the other rough rule of
thumb I have always used in my wasted career in industrial electronics
is to have the conductive plating 5 times the skin depth... So, 0.020"
would suffice for 10 megacycles...
Now, that begs the question for steel, or zinc plated steel... Anyone
interested can google up answers with a bit of personal effort...

What has always intrigued me though, is the concept that a moving
charge at RF frequences, spreads over the surface and penetrates only
0.020" the majority of the charge ( @10 mc ) , while still having lines
of flux penetrating radially to the electrical center of the metal
object... Yet, by the same token, if the metal shape is a hollow tube,
no signal will be detected upon the inner skin of the tube...

denny