Plywood is the greatest surface possible for graffiti. It will readily accept ink from a ballpoint pen, a Sharpie marker, a gel pen, or a rubber stamp. It will hold crayon, pencil, and charcoal. It can be scratched and carved with common and improvised tools, and is easily stained.
In Afghanistan, everything is made of plywood. Well, plywood and two by fours. The chairs you sit on, the desks you work at, the tables you eat on, the buildings you work and sleep in, the cubbyholes you stash your gear in, the smoke pit. All of it.
Walls are simple balloon frames with plywood on both sides. The roof is sheet metal over plywood over bare rafters. The doors are, depending on the purpose of the building, either a quadruple thickness of plywood, or stacked two-by-fours clad in sheet aluminum. Anything necessitating security beyond what is offered by plywood is built inside of a CONEX.
All common areas are covered in graffiti. And when I say covered, I do mean quite literally covered. Every available square millimeter is drawn on, written on, gouged, carved, with obscenities, names, dates, nonsense words, and truly talented art. Obituaries, promises, threats, dreams, out of context quotes, catch phrases, non sequiturs, mottoes, unit numbers, tours of duty. Penises. So many penises. Solo or in groups, erect, flaccid, and every stage in between. Realistic, cartoony, near-abstract. Every kind of penis. Freestanding or as a later addition. There was penis graffiti defacing other graffiti - beautiful, artful silhouettes of nude women, 40's pinups in three colors, and stick figures with cartoon breasts very often featured the later addition of a crudely scrawled penis.
There are generations upon generations of work. Ink fades, eventually, from the slow erosion of hands and asses, or from the sandy wind, or from the sun. In some places, there are ten years' worth of graffiti layered in ways that would make an archaeologist squirm were they to try to decipher it all a thousand years from now.
There are certain pieces of graffiti that are timeless and treasured. You can tell these because they are older than the other visible graffiti that surrounds them. You can tell they're older because they haven't been written or carved over, palimpsest-like; there will be a small but definite border of clean or scoured-clean wood around them, into which no later contribution encroaches. Sometimes, these treasured pieces have been carefully traced over many, many times, by people who want to preserve or renew them.
In at least one instance, some graffiti - a beautifully drawn Soldier's Cross - was preserved by a picture frame nailed directly to the wall. It had been drawn in an awkward place, about two feet up from the floor and nearly in a corner, the work of a Marine who had been laying at an awkward angle on a couch that had long since been trucked out to the burn pit.
There was a table in one of the hooches that had a huge "Afghanopoly" board game drawn on it, two or three times the size of a regulation Monopoly board. It wasn't intended to be functional, but could have been if you'd made the proper cards. The properties had all been named after various cities and bases, and had beautiful artwork. The Community Chest was called "Defense Budget" and the Chance cards were to have been called "Once more (into the breach)". There were five pieces drawn on the GO square - an MRAP, an Apache, a Kevlar helmet, a grenade, and a beer mug. The utilities were "Chow Hall" and "Shitters", and the Railroads had been replaced by a Chinook, a C-130, a C-5, and a caravan of camels. A piece of discarded bulletproof glass had been sized and cut and screwed down to preserve it indefinitely.
That same hooch was, incidentally, one of the very few hooches that had working A/C. One of the kids in the unit had been an unlicensed HVAC repairman before joining, and had cannibalized and manufactured enough parts to keep the cheap Chinese A/C unit mounted above the door working. You could sit in the big room, very still, and catch a faint cool breeze, or you could stand right in the doorway and have it blow directly down the back of your neck.
Another treasure bid you farewell on the way out. There was only one piece of graffiti on the inside of the door, so treasured that the other 18 square feet had been left unmolested in a sign of great respect. Someone had written, in mean, angular block letters, on the inside of the door at eye level:
"I hope they kill me while I'm standing here, so I can die happy."