Regardless of the reason you need a waterproof box, the project is no more daunting than building a wooden box and applying the right kind of finish. Making the lid watertight might seem a stumbling block, but ship technology answers the problem. Even if your carpentry skills are lacking, waterproofing the box is more about how you finish the box than the box's construction.
Draw the box and specify a dimension for its length, width and height. Transfer the dimensions of the box from your notes to 1-inch marine plywood, using a measuring tape. For example, if you want a box that is 7 feet long, 24 inches high and 36 inches wide, use a measuring tape to lay out two panels on the plywood, each 7 feet long and 24 inches high, for the sides. Then, lay out two panels on the plywood, 24 inches high and 36 inches wide, for the end panels. Then lay out two panels 7 feet long and 36 inches wide for the top and bottom.
Cut the side, end, top and bottom panels out of the marine plywood sheet using a circular saw. Sandpaper each of the edges of each piece, to provide a rough surface to which caulking can adhere. Attach one side panel to the end of each end panel with 1-inch wood screws. Attach two halves of the box frame together with the wood screws. Decide which side of the box is the front and which is the back.
Square the box by holding the end of your tape measure to one corner and pulling the tape to the most distant corner on the opposite side. Note the measurement. Repeat the process from the opposite corner. If the two measurements match, the box is square; if not, adjust the sides by pushing on the corners until the measurements match. Place the box bottom on the box frame and secure it to the box frame with screws.
Turn the box over. You'll need at least two strap hinges; for larger boxes, use one hinge every 12 inches, to prevent warping. Measure 1/4 inch from the barrel of each hinge and draw a line. Attach the first hinge to the back side of the box using 1/2-inch wood screws, so the line on the hinge is even with the top of the box's side panel. Repeat on all the hinges.
Attach the box top to the hinges using 1/2-inch screws -- often provided with the hinges -- so that its bottom sits 1/4 inch above the sides of the box. Caulk the seams between the box sides, ends and bottom with marine adhesive sealant.
Wait seven days, to allow the sealant to cure fully, then paint the interior of the box and the inside of the top with three coats of a spray-on rubberized coating, allowing the coating to dry between each coat. Spray two coats of rubberized coating on the exterior of the box. Allow the rubberized coating to dry thoroughly.
Cut a piece of 1-inch-by-1/2-inch solid, rectangular neoprene watertight gasket material equal in length to the perimeter of the box, using scissors. Spread a 1-inch-wide smear of marine adhesive sealant around the perimeter of the inside of the box lid, using a paint scraper.
Press the gasket material into place on the sealant and clamp the gasket material in place using one C-clamp for every 2 inches of gasket material. Attach the "hook" part of one standard vertical U-hook latch clamp -- one clamp for every 4 inches of box top length -- to the front edge of the box top using 1-inch wood screws.
Close the U-hook latch clamp, and hang the clamp on the hook so it hangs down on the front side of the box. Press down on the box top so it compresses the gasket material and use the mounting holes in the latch clamp to mark screw locations. Attach the latch clamps to the side panel of the box with 1/2-inch wood screws. Caulk all screws with silicone marine caulking.
- You're working with power tools, strong adhesives and potentially harmful paint. Read and heed all safety materials and instructions for their use.
- Before you cut the panels from the plywood, measure each panel again; remember the carpenter's adage: "Measure twice and cut once."
- To preserve the integrity of the gasket material when the box is in use, fasten the latch clamp nearest the center first, then work outward in both directions from there.
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