There is more than one way to convert an old wooden barrel into a planter. The barrel can be used whole, or cut horizontally or vertically. In most cases, you will want to retain the original material as much as possible, because that is part of the allure of having a barrel planter. If the barrel has a number of damaged slats, the best solution might be to cut it vertically, and create a half-barrel design.
Step 1: Mark the Barrel Top
Examine the barrel carefully and determine which is the best side. Since only half of the barrel is used for the finished product, bad slats can be replaced using pieces from the unused side of the barrel. Determine where the exact center of the top of the barrel is, and mark a line directly across that point, from side to side.
Step 2: Marking the Sides
Lay the framing square across the top to the barrel so that one edge is aligned with the center mark you just made. Extend that line down the sides of the barrel by tracing along the vertical arm of the framing square. If the square does not reach all the way to ground, use the level to extend the line the rest of the way. To check the lines, measure across the bottom of the barrel from the line on one side to the same point on the other side. This measurement should match the measurement of the same two points on the top of the barrel.
Step 3: Cut the Barrel in Half
Cut the top, bottom and sides of the barrel. In some cases, the barrel halves will separate and remain intact, but other types of barrels will fall apart when the cuts are made. If this happens, take slats from the unused side of the barrel and screw them into place as internal braces which hold the pieces in place.
Step 4: Reinforcing and Repairing Slats
Replace any damaged slats with slats from the other side of the barrel. Place another slat along each edge of the barrel, to help it hold its shape. If you plan to paint the barrel rather than leave it natural, it is now ready for painting. When painting naturally weather wood, the paint will soak up quickly and multiple coats will be required to get a smooth, even finish.
Step 5: Build a Simple Stand
Using 2 x 4 material, build a stand to support the barrel. Cut four short legs of the desired size, and then cut two side pieces which are approximately 3/4 the length of the barrel. Attach one leg to each end of the side pieces. Cut two braces 18 inches long. These pieces form the end pieces for the stand, and run between the legs on either end of the barrel, extending beneath the barrel. Paint the stand, if desired.
Step 6: Preparing for Use
If the barrel will be exposed to rain, you will need to cut or drill 4- to 8 3/8-inch drain holes in the bottom of the barrel. Place the half barrel on the stand, and add 4 to 6 inches of lava rock or some other porous material. Add 6 inches or more of soil. How much soil will depend on the plants being used, and the size of the barrel, but the maximum level should be at least 2 inches below the rim of the planter.