All 4 are designed to work on bare metals. 7007 is specifically for stainless Steel as it has a very strong bond. It is also suitable for all other types of bare metals, alloys etc. 7011 can also be used on the same materials but the bond is not as strong on SS. 7015 & 8102 are suitable for all types of metals including Stainless Steel, Aluminum, Aluminum and Alpaca alloys, Bronze, Brass, Chrome, Nickel, Titanium, Tungsten, etc but not gold.