We hear this question often all year long - both for Christmas and for parties!
Folks are building barndomeniums (yes, this is a thing), replacing traditional roofs with metal, designing tiny houses using shipping containers, and installing lights for the weekend on their RV.
Clearly, there is a need to install lights on metal surfaces.
While packing or duct tape may be a possible short term solution to installing lights on metal, installation for more than a few days in the outdoor weather requires a different approach.
For 18-AWG light cords - the type used during the Christmas season for screw-in C7 and C9 bulbs - our favorite and best solution is custom magnetic clips that fit onto the bottom of the sockets and provide a secure hold for the string.
These won't work with light strings that have a harness of 2-3 small wires. Make sure to test that your metal surface will work with magnets by testing that it has firm grip on a common refrigerator magnet.
See a sample of a patio lighting installation with magnetic light clips.
Adhesive clips manufactured for light strings are an installation choice for mini Christmas tree lights along the roofline or around windows and work on smooth metal or wood surfaces.
The clip part fits a standard incandescent light bulb. Keep in mind that some LED mini light strings have larger, non-standard bases.
The clips are shipped with the adhesive pads pre-scored in a large sheet. Just peel off a square and apply it to a clip. Not the most exciting part of an installation but one that is made fun with the addition of friends and possibly sweet tea or an adult beverage.
The adhesive hold is pretty significant - we always have to remove them gently and patiently with plenty of Windex and a blade.
These clips also work well on super-smooth tile.
Always test in an inconspicuous spot. Like with hair dye or facial acid treatments. (Women will understand this. Men .... think of pre-testing .... um... men never pre-test anything.)
For light-weight light strings or traditional mini lights, a great choice might be small suction cups with built-in hooks if the metal surface is freshly painted and super-smooth.
Keep in mind that suction cups will have a light hold and are best for short term installs. Be sure to test in an inconspicuous place if there are any concerns the paint might lift.
If all else fails, think zip ties. Not the world's most elegant solution but if your installation is on a structure with beams or pipes, this non-marring solution might be just the ticket.