I would like to add outside flood lights to my metal workshop building. Since all the wiring will come from the interior of the building, do I need to use special water tight wiring to install the flood lights on the outside of the building or can I use the flex cable I have been using to wire the inside of the workshop? Can I also place an outside electrical box on the side of the building using the flex? For both the electrical outlet box and flood lights I would be drilling a hole from the inside of the building to a wet weather box which would be flush mounted on the building. Thanks for all your assistance.

I would check your local electrical codes for guidance first. You can buy water-resistant electrical things. But, if the place might be sold eventually, things not being up to code may hinder the expedition of a sale.

Any raceway or wiring method installed outdoors must be rated for wet locations. This can be PVC Sch 40 with gasketed boxes and fittings, EMT with raintite compression fittings, or even as simple as UF cable, with weatherproof boxes and raintite connectors.

The flex cable you have inside actually may be rated to be installed outdoors, although ONLY if the cable is MC and conductors have THWN insulation.

If all you are doing is rilling the hole for a interior cable to pass through and connect directly to the box mounted on the outside over the hole, then the interior cable is fine, just seal around the cable before mounting the box.