First of all: Flaxseed oils vary widely in composition, including in the proportion of the most reactive triglycerides. Their polymerizing too fast, but not evenly throughout the oil coat, is thought to be a cause of the infamous flaxseed flaking. If your seasoning flakes, dilute your flaxseed with another, less reactive oil such as soybean.
Washing your flaxseed oil. Food-grade flaxseed oil will contain the lonnnnng polyunsaturated triglycerides that we want, but also other stuff: mucilage, phospholipids, flavorful short-chain fatty acids. They smoke easily but don't help with polymerization. In fact, they retard it and create gumminess, skins, and such. They conduce to rancidity in the unused oil, too. We want them out.
There are refined flaxseed oils manufactured for painters and woodshops, but I wouldn't trust my food to them, so we're going to have to do this ourselves.
Ethanol washing is very effective. 1 part each oil and 40% ethanol will emulsify beautifully and the ethanol will dissolve so much crap out of the oil. To break the emulsion, add 4 parts water, agitate, and settle. The aqueous phase separates overnight, but there's a particulate phase suspended in the oil which separates rather slowly.
(My "40% ethanol" here may be inexact. I diluted it from 95% ABV and I don't remember if I accounted for volume change. In ABV, the denominator is the actual volume of the solution; in v/v, the denominator is the sum of volumes of components measured separately.)
You can give it a rinse or two with water after the ethanol wash. This is better than aqueous-only washing in that it's less laborious and it does not pre-polymerize the oil as much, so it spreads thinner and doesn't cure as fast.
Washing with brine (a recipe: equal volumes oil and water, 100 g salt per quart of water) pre-polymerizes the triglycerides more than plain water does. Extended contact with the dissolved short-chain fatty acids pre-polymerizes too. Painters seek this. Ironmongers don't necessarily.
You want to avoid prepolymerization if you want the oil to be maximally wickable into crevices and between iron grains, if you want to put on thin thin coats (which have been particularly recommended for flaxseed oil), or if you want to use the oil for additional purposes (skin moisturizer, for example; has drawbacks if it's going to be in contact with clothing at all, though).
(The importance of thin coats probably comes from the need to get polymerization happening evenly through the thickness of the coat. See below about doping flaxseed oil with finely powdered food-grade red rust just before applying it, and about my accidental success with a first layer of soybean oil.)
Whatever your washing process, to help with the mixing, you can add inert ceramic chips or clean coarse silica (pool filter sand). After separating layers you may need to knock or sharply twist the container to shake loose any trapped globules. If the oil is turbid at the end because of suspended water droplets, you can clarify it with good old fashioned patience, freeze/thaw, or CAUTIOUS heating. Cautious because you don't want water to pool on the bottom of a hot pan under oil and burst into steam.
Prepping your iron. Scrub off whatever will scrub off. Electrolysis would be awesome if you can arrange it, but I was unable to get a suitable current source. If the outside of your pan retains some scaly bits, that's fine, but you may want to mix a little seasoning oil with limonene to thin it so it will penetrate the little crevices around there. Food-grade limonene is great to have around anyway, for certain kinds of cleaning. (But keep cats away from it and its vapor; like many essential oils, it is toxic to them.)
Your main problem is red rust, which is ferric oxide or iron III oxide, because it hydrates, expands, crumbles, and falls off, exposing more surface to oxidation. Black oxide or magnetite is a mixture of iron II and iron III oxides, and it's a sweet deal. Dimensionally stable, and a great surface for your seasoning oil to bind to. In fact, a better substrate than bare iron is; smooth bare iron doesn't offer much for the seasoning to hold onto.
Converting red oxide to black oxide can be done with steam, though you'll need an excess of it so that it will reduce your oxide and not just the oxygen in the air. Commercially it's done with reducing agents like hydrogen gas or carbon monoxide, neither of which I care to dick around with.
Doing this cyclically is a historic technique for passivizing metals, called "rust blueing": Form mixed oxides, convert what you can, card off the rest, repeat. Used by gunsmiths etc. Some chefs do this to carbon steel, and amateurs have done it with modest success on cast iron (YouTube, "The Step Most People Miss When Seasoning Cast Iron" by @noah-the-optimist).
(With the technique he used, he didn't like the outcome as much as a single-round process simmering vinegar in the pan. Comments on the video, including from the guy who taught him how to do it, indicate his technique could have been a lot better. The guy who taught him also says the vinegar process does not produce much magnetite relative to how much it damages the surface.)
Comments from @PrebleStreetRecords, a gunsmith, indicate that the rust blueing process could have been more successful. He recommends forming oxide with a 2:1:1 mix (by weight?) of vinegar, hydrogen peroxide, and salt. I presume the standard concentrations available at retail, 5% acetic and 3% peroxide. Convert red to black by pre-heating the pan, then inverting it over a pot of boiling water. The pre-heat is so the steam doesn't condense on the pan. Card off the remaining red rust with a Grobet carding wheel on a drill or bench grinder; softer than a wire brush, yet better at getting the red rust out.
But another attractive option is to phosphatize the iron. Very good anti-rust coating and an amazing substrate for the seasoning oil. Here we do want iron III, in solution with phosphoric acid. When the acid hits bare iron, electrons from the metal form gaseous hydrogen with protium from the acid, carrying both away. So locally the pH rises. This causes FePO3 to come out of solution and coat the metal.
The YouTuber didn't have great luck phosphatizing a pan with cola, as compared to forming black oxide on it, and he argued against it from material properties of ferric phosphate: hardness and thermal expansion.
He stated that ferric phosphate is undesirably softer and less durable, which does not accord with what I'm reading about conversion coatings from gunsmiths.
He stated that it has more temperature-related dimensional change than magnetite, but I don't think that's relevant and I'm not sure it's accurate. Both also probably have different thermal expansion than the cast iron itself does, but they're not a solid self-bonded layer, they're a layer of particles bonded to the iron underneath, and then we're surrounding them with an organic polymer a.k.a. plastic.
Also, the most important temperature related size change in ferric phosphate is from its water of hydration. If we mostly dehydrate it (heat it past 200 C), and soon thereafter we seal it under a waterproof seasoning coat, I think that's a non-issue.
At any rate, I have had great success using it as the interface between pan and seasoning, forming it not with cola, but with 1M food-grade phosphoric acid. That might account for some of the difference between his success and mine.
Some notes about doing the phosphate coating: You have to sweep off the hydrogen bubbles so they don't keep the acid from touching the iron. Later I hope to add an oxygen donor (nitrate?) to prevent gaseous hydrogen from evolving. It would also be nice to have extra iron III ions in the acid solution.
A plastic bendy straw is a usable bubble broom, with the long part as handle. A silicone tool ought to work too. Paper towel on the end of a stick has worked for me, but it does gradually liquefy, which I would bet is using up some of the acid, so it's not my preference.
This blog post is not about safety precautions for working with strong acids. Suffice it to say they exist, you should learn about them, and you should follow them. You're going to want a large volume of cool sodium bicarbonate solution available for cleanup anyway, so mix it and have it handy before you open any containers of acid.
Anyway, when it's done there's a nice grippy coating on the metal (wash & wipe it to remove loose particles, heat it to dry thoroughly) that drinks oil up thirstily. (So thirstily. If you've previously oiled bare iron or existing seasoning, oiling this stuff is going to astonish you.)
Oxygen donor: I've tried hydrogen peroxide (at drugstore concentrations), and it suppresses bubbling, but it also dilutes the acid. The reaction still happens, but it's much slower, which I don't like. But there's a potential oxygen donor I've been overlooking: red rust!
One mole of ferric oxide + two moles of phosphoric acid yields two moles ferric phosphate and 3 moles water. The evolved water raises the local pH, precipitating the ferric phosphate. Nice!
Rusting the bare iron with peracetic acid (add H peroxide to vinegar) preps it quickly. Just swab it on and let it evaporate. Repeat if necessary. You just need a dusting of red "sugar" rust over the metal, not bulk rust.
This will also produce some ferrous (black) oxide, which will not dissolve sufficiently to participate in the reaction. It stays attached to the metal. That's fine. The red oxide will dissolve in the aqueous acid, giving us just the iron III ions we need.
A gunsmith commenting on a YouTube video about rust blueing
OK, but can we add additional food grade iron III ions? Iron supplements usually contain iron II sulfate. We can bring it up to III with hydrogen peroxide, plus an acid (phosphoric would be the logical choice) to take up hydroxide ions. How to get rid of the sulfate? Maybe we can salt it out with calcium, but I'm not sure.
Food-grade calcium carbonate (sources: antacids, clean roasted eggshells?) will dissolve in an excess of phosphoric acid to give dissolved calcium phosphate. Calcium sulfate will not dissolve in water. But is there a pH where calcium phosphate is soluble but calcium sulfate is not? My chem is too long ago to tell me. You can get dissolved calcium in plain water with acetate, nitrate, or chloride, but that's just trading one anion for another. "Monocalcium phosphate" (calcium biphosphate IIRC) is soluble (2 g solute / 100 ml solvent) and is used as a food additive, so that's a possibility. I may pursue this direction further sometime.
Some folks recommend having the iron hot when you apply the oil. An experimenter and researcher on Reddit (u/VenetoAstemio in r/castiron) noted seeing no such step in 4 patents about seasoning, and that it might be a cause of the notorious "flaxseed flaking" if it is causing immediate polymerization before the oil can properly wet the surface.
Heating to drive out moisture (from between the iron grains, from the interstices of remaining red rust) is good, but if you've done it recently, a warm pan is probably better than a smoking hot one to apply oil to. And for flaxseed, "smoking hot" isn't very hot.
Another proposed cause of flaking, same source, is polymerization at the top surface of a thick coat proceeding faster than below, because of access to oxygen. Between what happens right then (shrinkage of the top surface) and what happens later (possible ongoing polymerization beneath, diffusion of unpolymerized oil into the top surface, polymerization of said diffused oil), tension in the surface could develop and tear it.
He found that suspending finely powdered food-grade red rust in flaxseed oil catalyzed polymerization all through a thick coat of oil, leaving it flexible and unflaky.
I semi-accidentally discovered a nice process that also combines the terrific glossy surface of flaxseed seasoning with a more flexible undercoat. I had a pan where part of the seasoning had gone weird through some complex combination of overheating, food burnt on, seasoning burnt off, what have you. Oil wasn't wetting it very well.
I scraped it flat with a razor blade, then heated soybean oil in it, pushing it around with a spatula. Shortly before it started smoking, it thickened and began to wet the weird surface the same as the rest of the pan. I took the pan off the heat and neglected it for a week, in which time the thickened soybean oil became gummy and very very tacky, and started to develop rancid odor notes. It had a promising resiliency, though.
I re-warmed it to reliquefy the gum, going nowhere near soybean oil's smoke point, and scraped out the excess with a silicone spatula. Wiping it off of the spatula was tricky because of how tacky it was; I succeeded by laying down aluminum foil that I could "butter" it onto.
Once I had the excess removed, the pan had a nice, flat surface with a little rubbery bounce to it. I raised the heat a little and briefly seasoned over that with a few drops of my washed flaxseed oil, to cover the tackiness with the famous glossy finish.
It may have been helpful that the pan was not extremely hot, that I did not use an excess of the flaxseed oil, and that I continuously spread the flaxseed oil around as it was cooking on. I wanted, and seemingly got, just the "skin" of a flaxseed seasoning, uniform within itself, atop the flexible underlayer.
I finished it by rubbing it with the inside of green onions, and heating until that started to come off again as a dark powder. I have noted several references from different times and places to using alliums for nonstick: Seasoning a wok by stir-frying green onions in it until they burn, rubbing a griddle with a cut onion before cooking dosas on it.
It is unknown whether that's just physical (spreading the oil on the wok, wiping traces of the previous dosa off the griddle) or whether there's some chemical effect. The reactive sulfur compounds might conceivably help to react off any still reactive moieties on the surface of the seasoning, so that they can't stick to anything else later. I'm not aware of hard research on that.
