You could try purchasing parafeen wax such as was used to seal jams and jellies. just be sure to drill a couple 1/2 inch holes in bottom of planters then carefull pour wax in and work around by tilting planter. linseed oil you can brush on. or you my try several coats of vegetable oil. you wont have the worry of the wax overheating and catching on fire. a burning pan of wax makes an awful black smoke and can be hard to extinguish. but if you feel safe melting wax, go for it. or be safe and use boiled linseed oil or vegetable oil.
if the planter boxes are cedar you can use as is. they will last for years. no matter what you use, be sure to drill drainage holes or your plants can easily drown , or die of root rot.