For all the powder-coated steel, solid hardwood and composite wood surfaces, general light cleaning with a soft cloth or soft sponge dampened with water should always be your first step. Avoid using abrasives, like scouring pads, since these are too aggressive and unnecessary. They'll leave uneven, concentrated scratches. Also avoid chemical cleaners of any kind on the wood surfaces; these can break down the finish and discolor the piece.
On the hardwood pieces, always wipe the surfaces following the wood grain and clean the entire piece evenly, even if the area needing attention is small. Localized cleaning can leave the surface looking splotchy and uneven. After wiping the wood with the wet cloth, follow up with a dry cloth since there's no reason to leave the surface wet.
If you've got a stubborn bit of funk on the steel surfaces or on the forming plywood, try a bit of soapy water or glass cleaner—one of these should do the trick. Remember: no abrasives!
AVOID HOT OBJECTS AND CLEAN SPILLS RIGHT AWAY
We're talking mostly about the hardwood here: as a general rule use a coaster or placemat under hot or sweaty dishes and glassware. The finishes we use have somewhat sealed up the wood's pores but by no means have they completely closed the wood off from the rest of the planet. We're not a plastic-coating, thick lacquer, bar-top kind of company. So since the wood is organic and will still be affected by heat and excessive moisture, clean up spills when they happen and don't put hot items right on the surfaces.
More on this: we've designed all of our hardwood pieces with seasonal fluctuations in temperature and humidity in mind. As you know, solid wood naturally will contract and expand with these changes. However, sudden, extreme changes in temperature or humidity could cause the wood to warp or crack so avoid this drama whenever possible.
ANNUAL CLEANING OF THE OILED HARDWOOD
For long-term maintenance, a deeper cleaning and oiling of the natural walnut and natural rift sawn oak (and to a lesser extent, the whitened maple) is recommended. While not strictly essential every year it's remarkable what a difference it makes if you do it regularly. The oiled surfaces develop a deep patina and luster that gets better and better looking, it makes the piece more spill and scratch resistant, and it's actually a very pleasant process. Plus it's amazing how good a freshly oiled piece looks.
Start by giving the piece a good cleaning with a dampened soft cloth as described above. (If the piece is seriously marred or soiled you can use a natural hair brush to clean the surface as a whole, with water. When dry, use some 220 grit sandpaper to lightly sand the entire surface, always working parallel to the wood grain. Remove the sanding dust with a non-marring vacuum attachment followed by a soft, dry cloth.) When dry, using a soft clean cloth, apply a small amount of Tried and True Varnish Oil (what we recommend), or, if you prefer, boiled linseed oil, following the instructions on the can (do pay attention to the danger of spontaneous combustion of oil soaked rags—it's no joke) and working parallel to the wood grain. Allow the finish to penetrate as recommended and remove all the excess oil, buffing the surface as much as you can bear. The more you rub the better.
Under certain conditions, the buffed surface may appear to sweat beads of finish for the first couple hours after application. This is normal. Just mop up these beads with a soft cloth and carry on.
For cleaning spills and stains of all sorts on the optional wool upholstery on our steel seating, here's a link to Maharam's maintenance guidelines.