In older pickup trucks the gas tank lived inside the cab behind the driver's seat. My '66 Ford had this kind of tank and needed replacing. Rather than scrapping the old tank I decided to turn it into a bench seat. The same idea could be applied to other smaller gas tanks to make different sized furniture. Please be sure to remove all gas vapors before working with gas tanks. Fire and explosions could result from being careless on this project.
Step 1: Cut the Tank in Half
My vision for this bench came from seeing that the tank's seam was diagonal from top to bottom. By cutting along the seam and then re-arranging the two halves, a nice L-shaped bench can be made. So, after you're sure there are no gas vapors in the tank, cut along this seam to divide the tank in two.
Step 2: Clean, Weld, and Bend
Clean up the two halves with some sand paper. How clean you go is up to you and what kind of finish the end product needs to have. I want this to be a little worn looking so I just took off the loose paint and scuffed the rest. After the sanding I folded the flange along the front of the bench down so it wouldn't stick out into anyone's legs. I used a crescent wrench and slowly worked my way back and forth until it was close to vertical, then finished with a hammer and anvil.
I welded the two halves of the tank together along the bottom edge. They could also be bolted together. If you're taking your time on the project like I did, make sure to primer over any exposed metal to prevent rust that would need to be sanded off later.
Step 3: Legs
There are many ways to do legs on a chair. I would have preferred to use some pistons and rods to keep the automotive style of this bench, but I had to work with the materials on hand. Using some 2" OSB wood I had, I positioned the bench at the height I wanted, placed the wood along the edge of the bottom tank half, and marked the angle to cut so the wood will match the bottom of the bench.
At this point it's also a good idea to think about making supports for the seat back. The weld and/or bolts along the bottom edge aren't strong enough. I used some 3/8" round bar bent in a semi-circle from the top of the seat back to the legs on the back side (not shown in any pictures, sorry). Something simple like a 2x4 could brace the top of the seat back just as well.
Step 4: Paint and Misc
With all the pieces fabricated it's time to paint them and make everything look nice.
After painting I attached my legs with lag bolts.
I added some posts made from bolts to the top of the seat back so I can hang cushions from them.
Depending on what you used for legs and how they are attached, you'll probably need to construct some cross braces between them. If you look closely in the first picture of this instructable, you'll see that I used two threaded rods between the legs to hold the bottoms from sliding out.
I hope this has given you some inspiration for up-cycling some old materials!