Alien Epoxy (Fallout 3)

Alien Epoxy is an item which is added to Fallout 3 in the Mothership Zeta add-on.

Uses
It can be used to increase the Condition of your currently equipped weapon, up to a maximum of 100%. The percentage repaired depends on the player's Repair skill:

Alien Epoxy comes in tubes and over 100 tubes can be found around Mothership Zeta. Since they have no weight, all tubes found can be carried around. While the effect of one tube may (or may not) conform to the table above, you can always increase a weapon's condition to 100% by applying multiple tubes one after the other, regardless of your Repair skill, which makes Alien Epoxy perhaps the most significant item which you get from MZ.

cannot repair armor or clothing.

Behind the scenes

 * An "Epoxy" is a step-growth thermoset polymer that solidifies around the applied area. This fits, suggesting that the epoxy fills in cracks or faults in the weapon.