First, determine the size and shape of the tank that meets your needs. Make your box to fit the size and shape that you need. Draw a sketch of your proposed tank(s) laying out the location and sizes of the holes you will need. Determine locations of the drain hole, sight glass and any other holes that may be needed for your custom tank.
Sizing a Tank
(Note: All dimensions in these calculations are in inches.)
Square or rectangular tank: Width x height x length divided by 231 = number of gallons
Round: 3.14 (Pi) x Radius squared x length. Then divide that total by 231 for a number of gallons. (radius = half the diameter)
Cut the Wood Pieces
An interesting observation I have made: Good money is spent for rectangular poly tanks and the first thing that folks do is encase the poly tanks with plywood. I chose to make the plywood tank and coat the tank with fiberglass inside and out.
Cut the parts, sides, bottom and top, corner braces, etc. and do a temporary dry fit. If you are sure of yourself then you can skip the dry fit step, but that is usually not recommended. I used ¾” plywood for this project.
Prepare the Fittings
Cut and make your plastic fittings – See figures 1- 4. These examples are for a sight glass made of 3/4” pipe and clear tubing (Figure 4). These same methods work for larger fittings too.
I used a piece of ¾ inch PVC pipe 3½ inches long, and 2 pieces of 1” PVC for the collars. 1” PVC will slide over the ¾” PVC making an excellent collar. Clean the ¾” pipe with PVC cleaner and then cement one collar flush with one end of the pipe. Allow the glue joint to dry. It should look like Figure 2 when finished.
Install the Fittings
Drill holes for all plastic fittings and install all fittings as follows. Apply a liberal coat of high-quality sealant to the fitting and insert from the inside of the wood panel being sure to fill all crevices with the sealant. Once the fitting is in place you can use a wet rag to smooth out the joint. This will be helpful later as you apply the fiberglass resign. I used a 25 year Red Devil Latex Sealer Caulk that I found on sale. Allow sealer to set. Apply PVC glue to the second collar and pipe and slide it into place against the outside of the plywood. (Figure 3) When the glue has set apply a smooth bead of sealant to the collar and plywood. Smooth the bead with a wet rag. The drain for the sewer hook-up is done the same way as the site glass is done. (Figure 5) Assembling the Tank When all fittings are done, starting with the bottom portion of the tank, assemble the sides and bottom using a good grade of glue on all joints and secure with screws. I used dry wall screws when I assembled mine.
Regardless of the type of screw you use, drilling pilot holes is important to avoid causing the wood to split or crack.
Install all corner braces being sure to use a liberal amount of glue and fasten in place with screws.
Cut some strips of fiberglass cloth 3 to 4 inches wide. Coat all the inside joints with a liberal amount of resin and lay the fiberglass strips in place, being careful to remove all air pockets and being sure to saturate the cloth. Then allow the resin to set. Once the resin has set apply a second coat overlapping the first coat by about 3 inches in all directions.
When you are satisfied that all the joints are sealed and the resin has cured you can apply the first of two coats of resin to the remainder of the inside of the bottom. Be sure to coat all surfaces including the edges. When the first coat has cured, apply the second coat and allow it to cure.
Now it is time to coat the outside of your holding tank with resin. My suggestion is to do the same with the outside as you have done with the inside, using a strip of fiberglass cloth 3 to 4 inches wide. Coat all the outside joints with a liberal amount of resin and lay the fiberglass strips into place, being careful to remove all air pockets and being sure to saturate the cloth. Then allow the resin to set. Once the resin has set apply a second coat overlapping the first coat by about 3 inches in all directions. Apply two more coats of resin to the outside and allow it to cure.
After the bottom has cured, fill tank at least two-thirds full of water and allow to set at least a couple of days to check for leaks. If you were not in a hurry and were careful about the application of the sealant and resin you should not have any leaks.
Apply a liberal coat of resin to all sides of the two tops and allow it to cure. Then apply a second coat and allow it to cure. Check the edges of the bottom for any really rough spots and sand if necessary to get a smooth fit. A liberal coat of caulking will fill most minor imperfections.
When the tops are cured, it’s time to install the large top.
Apply a liberal coat of silicon caulking to the top edge of the box and then screw the large top into place. Tank should be sealed at this point except for the smaller top.
Dry fit all piping attached to the top that will be feeding the tank. When you have completed the dry fitting, apply a liberal coating of sealant caulking and screw the smaller top securely in place. Once the top is in place and secured it’s time to paint the holding tank the color you desire.
To Prevent Freezing
A super simple way to prevent freezing (short of staying in the deep South from late Fall to early Spring) is to place the holding tank on a waterbed heater with the thermostat set to around 50 degrees. These are available new at swap meets, yard sales, used furniture stores, and e-bay.
Place the water bed heater directly under the holding tank and plug it into an AC outlet. Set the thermostat to the desired temp and forget about frozen holding tanks.
The Bus A 2” X 2”, a ¾” X 2” or L brackets (or combinations thereof) will hold the tanks in place unless you decide to do a bunch of wheelies.