We would recommend using raw linseed oil. It is entirely non-toxic, easy to apply and does an excellent job in prolonging the lifespan of your planter.
Just a word of caution when applying linseed oil. Liquid linseed oil in the can is no more hazardous than any other oil. But left-over linseed oil on rags, paper towels and so on has the unique ability to generate heat as it dries – sometimes getting so hot that it bursts into flames.