Anything you will do there, will end up staining the deck, because water will leak out on it, regardless. You can use deck stain on it, and that usually has a sealer with it. Nothing, is going to prevent eventual damage to your deck. Sorry. A better plan would have been to have this set up on the ground is an area, where they are protected.

"A better plan would have been to have this set up on the ground is an area, where they are protected."

And "find the right lumber". Several finishes claim standing up to elements but I haven't used one since older preservatives were banned.

You are in Chicago & it is August. If the shelving isn't built, I would return the lumber or store it for a future project. And re-think this. Because it won't get much use this year.

If the project is in progress: Think of it as disposable. Do what you can to extend the life of the unit. Treat all cut ends with something like Woodlife Paintable Wood Preservative.

I like to stand the cut ends in a bucket of Woodlife to allow the end grain to soak up all they can absorb. Double/ triple flood coat the flats of the wood. Top & bottom sides. Minimize penetrations and come back and drench those with more Woodlife. Allow the finished piece to season for 6 months or so, ideally out of the weather, and then come back with a penetrating exterior oil stain.

When building it, take care to crown the wood on any horizontal surfaces; Crown side up. You want any cupping to be on the underside.

Also, add sacrificial 'feet' to the stand. Small cuts of 1x or 2x wood (heart redwood/ cedar/pressure treated in the order) fastened to the end grain of the uprights where they contact the floor. Actually, the sacrificial feet don't have to be fastened if the stand can rest firmly on them. The idea is to raise the end grain off the surface of the deck to keep it from absorbing water by capillary action.

If you can, bring it indoors--garage--when growing season is over. Or sew up a Tyvek/ housewrap (breathable material) cover that you can cover it out of season.

Meanwhile, you can be planning the replacement. Think steel or aluminum frame. Keep an eye curbside/ rummage sales/ flea markets/ garage sales` for the frame. Maybe metal framed 'shelves' & use redwood slats??

I latch onto redwood in any form that I find it. Just stacked a bunch of it from a redwood planter that someone discarded. Finished disassembling it and cleaned up the wood. Cut off the ends where it had been stapled, cut away all the decayed areas, bundled & stacked in my shop. Destined to be another planter--maybe get to it this winter.

Just finished a cedar window box to replace a pressure treated one that lasted around 10 years.

Not quite completed here. I made liners out of heavy weight landscape fabric. Used mini blind slats in the hem pockets on top to keep the fabric from flopping. Redwood slatted bottom, resting on aluminum angle stock. Front cedar pickets and redwood bottom slats were scavenged material. I purchased the aluminum, a lengths of 1x8 & 1x4 cedar for the back, sides and trim. And stainless steel staples ($$ouch!!).

BTW, that 'ECO' wood preservative is, more than likely, just plain old 20 Mule Team Borax repackaged and with an inflated price sticker attached.

Tung oil with zinc. milk paint company sells it and the use you need is exactly in their description https://www.realmilkpaint.com/shop/oils/outdoor-defense/

Trailrunner, have you used that milk paint yourself? I love that it’s food contact safe.

I have a friend who uses marine spar varnish to protect outdoor wood / furniture, but I’ve never tried it.

There is no protection for using the wrong materials for the job. Not unless you want the work schedule of a 1930’s sailor who started at one end of the boat stripping and painting until they got to the end. Then started over again at the other end again. With lead based products.

Telephone poles are made from ordinary pine lumber (in a nicer finish grade). They are treated with Pentachlorophenol, CCA (contains copper, chromium and arsenate) or creosote. If you find any of these preservatives your pine wood will last for years, but finding uncut pure white heroin will be easier to find than the three listed above. Another preservative used to treat Telephone poles is Copper Naphthenate and is not restricted. It can be purchased and found in stores but won't look pretty on your plant stand.

Water doesn't rot wood, microbes, fungus and bugs eat the wood and what rot is, and these little organisms need water. So you need something that will kill these little critters and/or make your wood waterproof. Thompson water seal IS NOT a preservative and nearly worthless sealing out water.

My suggestion is soaking the wood that will touch the ground in Copper Naphthenate, then buy a good wood preservative and sealer to apply for anything above ground. TWP is registered as a preservative and sealer and registered as a preservative . TWP has high VOC's and is banned in about 20 nanny states because of the VOC level. Illinois is one of the states banning TWP, and so does Indiana so you would need to drive to Wisconsin to find TWP. (I need to drive to WI to find good fireworks)

Many stains are not preservatives and the heavier they cover the wood means they are more like plan old paint. Take the time to read the fine print on the cans and make darn sure what you're going to use will KILL the microbes that cause rot. what ever you use. Look for the word COPPER.

@party_music50 I have used marine spar varnish and it is useless if it is on a flat surface like a table and bench or floor. It says this somewhere on the net and I found it AFTER I had spent hours using it. One has to use a product that is meant specifically for something that will be out 24/7 in sun and water and is also a horizontal surface . The product from Milk Paint is tung oil and every woodworker is familiar with its attributes. The addition of the zinc is what I am after. I trust the hundreds of reviews and the company. Will let you know how it goes. Have to wait til the rain next week is over. c

Several years back, I experimented using tung oil on the redwood side table of a gas grill. Within a couple of years (central Wisconsin weather) the tung oil turned black and not an appealing black. Perhaps the addition of zinc might enhance the performance and prevent the finish from turning ugly.