A pizzeria near my apartment in Bushwick needed indoor and outdoor bench seating for their clientele. Being a charitably-inclined guy, as well as a starving artist, I was happy to put something together in exchange for free pizza. Since the budget was so low, my materials were limited to steel and construction-grade lumber. The original plan was simple: a rectangular steel frame with an 8-foot-long seat made from wooden planks. Unfortunately, the lumber I wanted was unavailable because of Hurricane Sandy-related repairs, so I substituted sheet metal.
The problem with steel sheets, especially in such large dimensions, is that they're sensitive to heat and warp easily during welding. The most common way to combat this is to make small spot welds instead of a continuous line, "stitching" the sheet to the frame.
The first sheet looked great spot-welded to the frame, but I was worried about the sharp edges. There are a lot of families with small children in my neighborhood, and toddlers are very inventive at finding ways to hurt themselves. So after I made the spot welds, I seamed* the sheet metal around the frame on all sides, then ground the weld down to eliminate any sharp edges.
This DID cause the sheet metal to warp, but in lucky way. Since the welding heat was evenly and consistently applied, I got an even and consistent ripple in the sheet. Totally unintentionally, I had created seats! These benches weren't designed for ergonomics, but they were noticeably more comfortable after warping.
The final step was sealing the benches to prevent rust. I used a self-priming matte black paint from McMaster-Carr, supposedly very durable and water resistant. It didn't stick to the metal as well as I'd hoped (one of my shop buddies told me "no matter what it says on the can, use a primer"). On the other hand, even the bench that goes outside will be covered by an awning, and some slight surface rust will probably enhance the look of the pieces. Honestly, if I'd had the budget for the chemicals I would have pre-rusted the benches (similar to Brooklyn's new Barclays Center). Controlled surface rust can protect the underlying metal, and eliminates the need for other finishing and painting.
I'm glad I had a chance to work with sheet metal in such large dimensions (even if I would have preferred wood). The total materials budget for each bench was under $100 - not bad considering the scale. And these benches are going to go through some heavy public use, so I'll be able to check on their durability during my year of free pizza!
*In basic terms, I melted the edges of the sheet metal into the steel of the frame, fusing it into one piece without adding any welding rod.