Most of our pans are made of 2.5mm or 3mm pressed carbon steel, they aren't treated save for light coating of oil to help prevent rust during transport and storage, the grey colour is simply the colour of the raw steel. It might seem obvious but it is still worth reiterating that until the pan is ready to be used, until it is seasoned it should be kept absolutely dry and the protective oil left in place. If allowed to get wet, or stored damp, this is the time that the steel is most susceptible to rust as the pan is yet to develop its own protective coating.
Does it matter if it rusts? Not really except for the sake of appearances, functionally it shouldn't make a difference - indeed there are those who deliberately rust their pans, blacken the rust by boiling, and then season believing it results in a truer, harder surface than the simpler method described below (albeit not without risk). If any rust does appear probably the best advice is to scrub off what you can with a scouring pad, a copper cloth, or brass wire brush; I'd probably avoid wire wool, or sand paper as these are likely to either polish or roughen the surface. The result will be slightly patchy, but as the seasoning builds and the pan blackens any uneven colour will gradually disappear.
So having kept the pan dry until you are ready to go, the first thing to do is to get the pan wet. You need a bowl of very hot, clean, soapy water, a sponge or a soft washing up brush, with perhaps some extra detergent on the brush or sponge itself. You need to wash away all the protective oil that's on the pan as it is the wrong kind of oil, it's a non-drying acid free oil most likely a light mineral oil of the type commonly sold for treating chopping boards, and used because it doesn't go rancid, evaporate, corrode the steel, or degrade. Despite these virtues it isn't what you want so you need to thoroughly, very thoroughly and gently scrub it away, and when you think it's all gone you should change the water and wash it all over again. Then having done this you need to rinse, and rinse, and rinse until you are absolutely convinced that no trace of soap (or oil) remains.
An obliquely lit, approximately 50x magnification image of a deBuyer frying pan's surface, after cleaning but before the application of any oil. You can see the grooved and pitted nature of the raw machined steel. Ideally any protective coating and swarf should be cleaned from all these depressions prior to seasoning. You can test the cleanliness of the pan by wiping with a lightly oiled tissue - if it comes away at all grey, it might be worth washing again before seasoning. Once clean though the microscopically uneven steel should allow the seasoning a better key.
Sufficiently convinced the pans should be immediately dried with a clean cloth, and placed over a low heat or in a hot (450°F, 230°C) oven for at least ten minutes to drive any residual moisture away and out of the steel's surface. There are some who think it beneficial for the pans to then be baked dry at this temperature for an hour, which should slightly darken the surface possibly indicating the development of black iron oxide and potentially resulting in a better fix for the seasoning. There may be some truth in this, and it certainly won't harm your pan, but our feeling is such lengthy pre-seasoning can be considered optional rather than necessary.
Either way, once your pan has sufficiently cooled to touch (please do be careful here) you'll want to rub in the oil, but before you do it's worth paying a little consideration to the type of oil that will provide the best results, as it might not be what you think, [traditionally it would have been lard, dripping, or tallow] and almost certainly will not be found in your kitchen cupboard. The natural tendency when seasoning pans is simply to grab whatever is the least favoured or cheapest of your cooking oils, slop it around the pan, and heat until it burns, with patchy, splattered, uneven results. The pan will probably work moderately well and slowly blacken to a more even finish, or at least not be irretrievably damaged; but it's not the best way. Almost without exception culinary oils are non or semi drying oils and are used in the kitchen for precisely this reason - being less prone to rancidity in storage or coagulation during cooking: certainly virtues for a cooking oil. It is however these very characteristics that do not lend these oils to seasoning iron. The best seasoning oils are drying oils, that is readily oxidising and tending to polymerisation, oils familiar to woodworkers, painters, and restorers, of poppyseed or linseed, both of which can be found in edible form.
Of the two it’s probable that linseed (or flax oil) provides the more durable finish, that at least being the prevailing belief amongst painters and probable explanation for poppyseed oils infrequent use as a wood finish despite its far lesser tendency for yellowing with age. It’s also likely the easier to acquire, and certainly the one we would recommend. Whatever you decide to use the next step is the same, to rub the oil all over the body of the pan, over the inside working it thoroughly into the steel and up to the lip, not over the handle, but certainly if you are seasoning over a gas flame or in the oven over the outsides and underside of the pan, as this should grant the raw steel some further protection from corrosion - obviously if using a range or electric hotplate the mess this could incur might be considered unwelcome, and perhaps not worth such marginal prophylaxis.
Either way we want next to take some kitchen towel, tissue, or a lint free cloth and to carefully, evenly, wipe away the oil until the surface is almost dry. At this point any streaks or variability in the coating will become burnt onto the pan as darker lines, splatters, and uneven colour - nothing that will affect functionality more a slowly fading reminder of a job that could be better done. Now the pan needs to be heated, either in a hot oven (in excess of 450°F, 230°C) or more likely perhaps given the dimensions of some of these pans a medium to high hob. I think the best advice if using an oven is to put the pan in cold and upside down, close the oven, turn on the heat, open a window or even an outside door, close any interior doors, and go away to read a book, watch a movie, catch up on some paper work. Have a quick check in the kitchen every quarter of an hour or so, and after a couple of hours switch off the oven and allow it to cool. Now at this point I’d say it’s done, or at least it will work pretty much as it should, however it has been suggested that the oiling and heating/cooling process be repeated a further four to six times which will certainly give a better looking result possessing the blackness of aged and well seasoned pan. Using a hob it’s best to try to heat the pans as evenly as possible, which means using the correct sized ring, starting cold, and slowly raising the heat as the pan warms up, this should result in the most even darkening of the steel. Once smoking the temperature can be reduced to a low medium heat and the pan left for twenty minutes or so, or perhaps as long as you sensibly dare. Once cold the oiling and heating process can be repeated which will again result in a more satisfyingly even colouration, and perhaps a marginal improvement in performance.
Well that’s the beginning at least, the important thing now is to use the pans, and to allow them to turn black in the months and years ahead.
Obliquely lit, approximately 50x magnification images of two deBuyer carbon steel frying pans. The image to the left is of a freshly seasoned pan, the same one photographed above. You can see already that the parallel machining grooves have been filled and the pan exhibits a darker, speckled, but more even surface. On the right we have a different pan which has seen regular use for in excess of five years. The finish is certainly darker, the grain slowly being filled resulting in a smoother, increasingly non-stick, and more scratch resistant surface. To the naked eye it appears pitch black with a satin sheen.