Sealing the inside of a cedar flowerbox?

http://homeguides.sfgate.com/waterproof-wood-planter-box-44260.html

Cedar should not rot. But remember you have to be able to let access water drain through.............

If you waterproof the inside, you may end up with water pooling inside if there are no drain holes. I would consider using a marine grade type polyurethane that is made to be in damp conditions and not deteriorate fast. Make sure that you put in a few holes for drainage and that the cut edges are coated also. I would go ahead and do the outside also. Unless you eat the flowers, it will not harm the plants.

I would line it with the coconut mat - it protects the wood, and allows the roots of the flowers to breathe. You can also cut up baby diapers and put plastic side down on the bottom. It holds the water for the flowers but keeps it from sitting on the wood.

I've used roofing tar very successfully for many years. I especially like that the black tar blends with the dark soil. It sets off whatever color flowers I have that year and doesn't look messy if the soil splashes or gets uneven over time. It may not seem like a big deal to most people, but my grandkids love to water the window boxes and it turns into a big deal! I'd rather have them learn to love gardening than have perfect boxes, but still! Also, the tar seems to keep cats from sleeping in the boxes. Of course, you wouldn't want to plant anything edible in a box lined with thick tar, but flowers don't mind. You do have to add another layer of tar every 4-5 years, but the soil has to be replaced anyway, so It's not a problem. The tar is sold at farm supply stores, home stores, and Wal-Mart. I recommend buying at least the gallon cans (It's cheaper), and you'll go through more than you think smearing it on nice and thick. Use a flat piece of plastic or wood that you can throw away when you're done. Extra tar in the can lasts forever if tightly sealed. Hope this helps!