Wash the surface of the cast iron planter inside and out with mild, soapy water. Dry thoroughly. Use a zinc-rich (90% zinc or more) undercoat paint.
Paint on a rust-resistant exterior paint with an epoxy-resin base. Let dry and apply a second coat.
Apply a clear sealer formulated for exterior use if you want to keep the natural look of the cast iron. Coat every bit of the cast iron planter both inside and out. Make sure the sealer protects against UV rays.
Fill areas where water can pool with epoxy resin after painting or sealing the planter. This keeps standing water from wearing down the paint or clear sealant, which can cause rust.
Bring cast iron planters in for winter. If they must be left outside, then cover with plastic.