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How to build a geocache

Parts I used to build this geocache:

1 gallon plastic jar. The opening is 4" and is approximately 10 " tall.
Plastic tray. This is the bottom half of a plastic container that food came in from the grocery store. I cut off the top half of the tray (which will be used to build another geocache) and cleaned it out. This tray will be used as a recess for the lid of the jar. When the geocache is sitting on the ground, the jar will be upside down, and this will keep the lid off the ground. Better shown in pictures below.
Another view of the plastic tray.
Plastic jar and plastic tray together.
6 pound bucket of quick setting concrete. I wanted to add ballast to this geocache as it would be in a windy place. For the geocaches that I will leave in protected areas, such as in the mountains, I will not add concrete ballast.
Expando foam. To add bulk to the geocache rock, I sprayed on expando foam as filler.
Hardware. Any size will do, but I used (4) 10-24 bolts 1-1/4 inches long with 2 washers and hex nut on each. The purpose of the hardware is to help anchor the ballast or the expando foam to the jar. This step is no doubt overkill and not necessary, but I like the added assurance that the jar will not pull out of the core.
Fiberglass skin. To protect the expando foam, I use fiberglass as the outside skin. Shown is the fiberglass mat, fiberglass resin and hardener, and cheap throw away brushes to paint on the fiberglass resin.
Fleckstone paint. This spray paint does not spray on in a single color, but is speckled. It has the appearance of marbled rock. It comes in various colors and is perfect to give the appearance of natural stone. It can be found in hobby stores or hardware stores.
Concrete mixing bowl and whisk. I mix the concrete in this bowl and use the whisk to help stir the concrete mix. It has a wide opening and is easy to mix the concrete. Not necessary if concrete ballast is not being used.
Popsicle sticks. I use these to mix the fiberglass resin and hardener. I also used them as anchors to hold the expando foam to the concrete.
Latex gloves. When applying the concrete mix to the jar and when working with the fiberglass mat and hardener, a must to protect the hands. Harbor Freight is an excellent source for these. I bought a box of 100 for under 7 bucks.

Building the concrete base.

I set the jar on top of the plastic tray and drew a circle the size of the opening. Even though I used a black marker on the black tray, I was able to see the outline no problem.
Here is the opening cut out. The jar will fit down into this hole.
Jar set down into the hole. The purpose of the tray is to leave a recess that the jar lid will set into. This allows the lid to be removed and to have the lid not sitting directly on the ground. This will become more evident further on.
I drilled 4 holes into the jar and installed the bolts. The bolts will act as an anchor to hold the concrete secure to the jar. Even if the concrete ballast is not used, the bolts will help secure the expando foam to the jar. Probably not necessary, but I like the added assurance that the jar will be securely held in place.
Jar with bolt anchors set into the plastic tray. The jar is just a friction fit into the tray. I did not epoxy the jar to the tray, though that certainly wouldn't hurt.
Concrete ready to mix.
Concrete mixed up and ready to apply to the jar. The consistency is a bit on the thick side. I want to be able to mold it around the jar and have it hold it's position while curing.
Here the concrete has been molded around the jar and the plastic tray. The bolt anchors are firmly embedded into the concrete. The popsicle stick have been stuck in at an angle to help secure the expando foam to the concrete and to the jar. I let the concrete cure for 24 hours before going to the next step.
I then cover the jar with expando foam and let it cure overnight. This helps to give a rock-like look to it.
I then cover the foam with fiberglass mesh and resin. This helps to fill in the voids and give a smoother, more natural look. Be sure to wear gloves when working with the fiberglass resin.
Spray on a coat of fleck paint, using the color of the rocks local to where the geocache will be placed. Here is the final geocache ready to be hid.


Here are some other geocaches I have built. Some
of the techniques differ slightly then shown above.

The next 3 pics are of the same geocache. I did not use expando foam for a core on this one. The container was small enough that I simply covered the entire jar with concrete.
The next 3 pics are of 2 geocaches that have an expando foam core. The skin was made with muslin strips and plaster of Paris instead of fiberglass mesh and resin. Not sure how long the plaster of Paris skin will hold up in the weather, but will find out.


Building your own geocache is certainly fun, however, there are plenty of commercially available products online that can be turned into a geocache. Here are a few items I have found that would make excellent geocaches:

This simulated-rock is intended to cover outlets and other unsightly boxes in your garden. Made of polyresin, it can be modified as you see fit to further customize your geocache. You can further help blend the rock into the surroundings by scuffing it with moss, or using some speckle-paint or other paint patterns.
Here is another example of a faux-rock intended for garden and landscape use that can easily be adapted into your own cleverly-designed geocache.
Of course, not all of you geocaches need to look like fake rocks. These waterproof survival capsules make perfect geocaches that can be hidden anywhere. The eyelet on the capsule lid makes it the ideal candidate to hang from a tree, and its slender profile is good for easy concealment in most surroundings.

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