waterproofing a wooden planter

Would a small trash can fit by some odd chance?

Fiberglass resin, thin, painted on with a brush. Could put in a few small PVC couplings or similar in holes in the bottom so that excess water can run out without rotting the wood.

Not that I can see. These are about 14 inches wide and 32 inches long, sort of the shape of a horse trough, if you remember those from old westerns.

So get a larger bag and cut it down.

They range in size up to those big honkin' heavy duty 55 gallon mf'ers...or outdoor leaf/yard debris bags or even those contractor cleanup bags which are even bigger and heavier gauge ;-)

Wife likes that idea very much. We never worked with fiberglass before but it seems similar to epoxy.

Found this one on Amazon

Might work but I would put in some fiberglass scrim to keep it intact from thermal expansion and contraction.

I had to look up what is scrim. haha thanks

Thanks. There is no rot at the moment, just the usual small gaps where the boards are put together. Besides, that stuff looks expensive.

She's opposed to any kind of plastic liner, such as garbage bags or the heavy plastic that we have in a roll. She wants something that she won't accidentally rip or poke holes in as she's gardening.

It is epoxy. You can find it at HD, Walmart, etc

Build your flower boxes out of PVC and you wont have to worry about rot.

I have never seen that in 4 x 8 sheets ! .. so I checked-out Lowes <Canada > ...

Many years ago I had a redwood picnic table on my deck that started to rot in places so I took it apart and made a planter out of it to grow tomatoes. I had lined it with polyethylene to keep water in. It lasted several years but eventually rotted out as I think eventually any wooden product would. OP wants to use what he has and should at least get a few years use. I ended up with large plastic pots that lasted a long time and were still in use until I got my new Trex deck when I decided to remove them.

Cedar or treated wood planter boxes should last for years if sized to to hold a plastic planter - with breathing room between the plastic planter and the sides & bottom of the box. . and, of course proper drainage holes in the bottom. This method seems so much better ; long-lasting ; and easier on the gardener - less fuss. John T.