I'm not a conservator, so I cannot give you a definite answer. And the answer will depend on the value of the object. If this is a rare remnant of history, do not attempt any restoration and let a professional restorator do their work. In that case, it would be almost illegal to apply epoxy to it, because the process isn't reversable. Most amateurs trying to conserve objects do more harm than good. If you just want to avoid buying a modern giant flower pot and there are hundreds of such vessels left, you have more leeway.
As pointed out in the comments, I would avoid power tools. Since the object is already brittle and crumbling, you might damage it with a power tool or it might sand away much more of the object than you like. A metal brush or sand paper would be the safer alternative, even if the cleaning takes a little longer. And please wear a dust mask and eye protection to protect yourself against dust particles and things like mold spores and bacteria that may be contained in that dust.
To restore it to a fresh look, I would apply a thin layer of fresh clay, which you can buy in many craft stores or online. The pot looks like it may have been made from red or yellow clay, so I would stick to that color. If possible, put the pot on a turntable, so you can rotate it like a potter would do on a potter wheel. In a big bowl mix the fresh clay with enough water to make it a thin paste the consistency of toothpaste. Then transfer the clay paste onto your vessel with your hands and smooth it out while turning the vessel. Let the vessel dry at least 2 days to make sure all moisture is removed from the inside of the walls. The disadvantage of this method is that the clay dissolves if it comes into contact with water again so you absolutely need to seal it.
An alternative is to coat the vessel with a layer of crafting cement. That still absorbs some water, but in general it's more resistant to water and doesn't dissolve again. The disadvantage of this method is the grey color of the cement and you need to wear gloves while working with wet cement.
This fresh layer of clay will look much more authentic and natural than a layer of paint. Paint always smoothes out the little irregularities of the surface and makes things look flat and monotone.
As for the conservation, the main problem is that the clay soaks up water (the original clay of the vessel just as much as the fresh clay). That causes gradually more damage (by lichen, bacteria and other microbes), but it also poses a challegne for the conservation, since the pot will soak up vast amounts of any liquid (like epoxy) you apply to it.
There are ceramic / flower pot sealers available online as a liquid to paint on or as a spray, but those are meant for small objects and you need to apply several coats. Given the size of your pot, that might become expensive. An alternative is brickwall sealer, which is usually available in bigger quantities and also designed to seal absorbant materials. Remember to look for matte finish if you don't want your pot to sparkle in the sun.
A more natural sealer is plant oil. You simply soak the vessel in as much oil as it can absorb and let it sit. Over time the chemical composition of the oil will change and it first becomes sticky and then almost like plastic. It's exactly the same process as in oil paints. I assume that the spills you see in one of your pictures are actually oil spills. Usually (for cookware) you would heat the oiled pot to accelerate this process, but I assume that's not an option here. If you cannot heat the vessel, it will take several months to completely change the oil. The disadvantage of this option is that the oil gets very sticky before it becomes inert. It might hold onto dirt in that stage.
I think epoxy would be problematic for such a project for several reasons:
- You need a very liquid consistency of epoxy so it soaks deeply into the clay to stabilize it. That means you need a lot of epoxy, which can become expensice very quickly.
- You need an epoxy with a long drying time to be able to cover the pot evenly. That poses the risk of leaves, insects or other dirt falling on the sticky epoxy and ruining the looks.
- Depending on the type of epoxy you use, it can heat up and harden extremely quickly if you mix big batches.
- No epoxy is 100% UV resistant. They will all yellow over time, especially when exposed to direct sunlight.
- Epoxy is a health hazard. You need to wear gloves, eye protection and in many cases a respirator (if working with big amounts of epoxy).
- Epoxy is an environmental hazard. Any liquid epoxy and hardener that you don't need any longer must be disposed of as hazardous waste.
- Epoxy always has a smooth, shiny finish. If you want a matte finish, there are better alternatives (like sealants).
