Sealing threads on geocaches: How to keep the cache dry
The most reliable method for sealing the thread of a geocache screw cap is a precision-fit O-ring combined with silicone grease. This combination keeps rain and moisture out permanently, can be checked in seconds during every service, and the O-ring is easy to replace if worn. Glue or heat-shrink tubing may provide short-term help, but they are not a real alternative to a maintainable seal.
In short:
- A precision-fit O-ring combined with silicone grease provides the most reliable and maintainable seal against water and moisture ingress.
- The O-ring should sit in a groove that is 15 to 20 percent shallower than its cross-section to ensure an effective face-seal.
- SLA and SLS printing methods provide waterproof enclosures significantly better than FDM printing, which additionally requires post-processing to improve.
- Hot glue and heat-shrink tubing are only temporary solutions, as adhesives become brittle and capillary forces can let moisture in.
- Regular maintenance, especially re-greasing the O-ring, increases sealing performance and extends the container's service life.
Table of Contents
- Sealing threads on a geocache: The O-ring as the standard solution
- Which O-ring size fits PETlings and 3D prints?
- Which printing material really keeps moisture out?
- Why glue and heat-shrink tubing often fail
- Maintenance checklist for your next cache service
- Publisher's perspective: Why O-ring solutions are winning out
- Suitable products for a watertight hideout
- Sources
- FAQ
Sealing threads on a geocache: The O-ring as the standard solution
An O-ring solves the problem that causes most leaky caches: tiny gaps in the thread through which water enters during rain. Unlike a layer of glue, the O-ring remains elastic and adapts to the thread flanks every time it is closed. This makes it the only solution that can be opened and resealed any number of times without losing effectiveness.
You don't need much for installation: a suitable NBR or silicone O-ring, some silicone grease, a clean cloth, and ideally, fine tweezers for small PETling rings.
How to proceed:
- Clean the thread and groove with the cloth to remove sand, dust, and old grease residues.
- Carefully place the O-ring into the intended groove without twisting it.
- Apply a thin layer of silicone grease to the O-ring and thread; more is not necessary.
- Screw on the lid with even pressure, without tilting the ring.
- Finally, a visual check: if the ring is visibly squeezed out anywhere, it is seated incorrectly.
A simple test shows whether the seal actually holds. Hold the closed container upside down in a bucket of water for a few minutes or press it firmly under running water, then check the logbook and interior for moisture.
Pro tip: Apply the silicone grease very sparingly. Too much grease attracts sand and debris outside, which then work their way into the sealing surface and cause exactly the opposite of what you want.
Which O-ring size fits PETlings and 3D prints?
The groove design determines whether an O-ring really seals or just sits loosely. As a rule of thumb, the groove should be 15 to 20% shallower than the cross-section of the O-ring, as documented by Formlabs in its tests on watertight enclosures. As a result, the ring is slightly compressed when closed, forming a so-called face-seal, where it sits flat between the lid and the housing and is hardly exposed to shear forces.
For standard PETlings with common thread diameters, you will often find a suitable groove as standard, which is why usually only a replacement O-ring is worth it here. With custom-designed or 3D-printed lids, however, the responsibility lies entirely with you:
- Generally, it is better to design the groove depth just a little too shallow than too deep.
- For FDM printing, plan the groove diameter one to two tenths of a millimeter larger, as plastic shrinks slightly when cooling.
- After printing, smooth the edges of the groove with fine sandpaper so that the O-ring does not tear on rough layer lines.
Important to know: The 15 to 20% shallower groove is a guideline for most common cache sizes, not a rigid standard. For very small nano-containers or unusual ring cross-sections, it is worth doing a test run before the final version. If you are unsure when buying, you can find concrete measurement references in a guide to the correct PETling O-ring size and in a short guide to the Bison O-ring dimension.
Which printing material really keeps moisture out?
Not every 3D-printed material is equally suitable for a permanently weather-resistant cache. FDM prints from PLA or PETG are inexpensive and easily available, but the layer structure makes them inherently more porous than other methods. Prusa recommends at least four perimeters and a high infill density for watertight components in order to reduce the number of microscopic channels between the layers.
If you want to play it safe, post-process FDM parts additionally:
- A thin epoxy layer on the sealing surface reliably seals fine pores.
- Media blasting smooths the surface and improves contact with the O-ring.
- A second coat of clear resin is particularly helpful for heavily structured exterior surfaces.
SLA and SLS printing are clearly ahead in terms of tightness. Formlabs shows in their own print tests that SLA and SLS components withstand significantly higher pressures than FDM parts, because the manufacturing process leaves virtually no open layer transitions. For a cache that is to stay outdoors for years, this is an advantage that pure FDM only compensates for with extra effort.
Self-printed O-rings from TPU, on the other hand, are at best a makeshift solution. They only work reasonably well with very soft TPU, but they lose their elasticity faster than an industrially manufactured ring made of NBR or silicone.
Why glue and heat-shrink tubing often fail
Many cache owners reach for hot glue or silicone sealant first because it's quick. In the long run, this is rarely the better choice. Adhesive becomes brittle, cracks with temperature changes, and it is precisely at these cracks that moisture is drawn into the interior by capillary action, often unnoticed for weeks.
- Glue and hot glue: A quick solution, but prone to cracking and practically impossible to repair without completely removing the old adhesive layer.
- Heat-shrink tubing: Good against curious fingers and as theft protection, but not a real sealant for the thread itself.
- Double container or internal hide: A sensible additional security measure, but it does not replace the primary seal on the cap.
Reports from the geocaching community on PETling inserts also show that the containers themselves are usually quite tight. Problems arise more frequently due to an incorrectly installed seal or aged material than from the basic housing. So, if you already have a good container, you often only need to address the lid seal instead of looking for a completely new hideout solution.
Maintenance checklist for your next cache service
A cache that was once tight does not automatically stay that way forever. Silicone grease loses its effect over time, especially in locations with strong sunlight or frequent contact with sand.
- During every service, check the O-ring for cracks, hardening, or deformation.
- Always renew the silicone grease, even if the ring still looks intact.
- Perform a quick tightness test with a water bottle or damp cloth on the closed lid.
- Make a brief note of the result in the logbook or owner note so you know when it was last serviced on your next visit.
Pro tip: Put together a small service box with replacement O-rings in common sizes and a tube of silicone grease. With this, you will be done in a few minutes on every maintenance tour, instead of realizing at home that the ring is missing.
Publisher's perspective: Why O-ring solutions are winning out
In specialized shops for geocaching, we work with weather-resistant hideouts every day, from the classic film canister to the 3D-printed custom container. What is noticeable here: most complaints about wet logbooks have nothing to do with the base material, but with a missing or incorrectly seated seal on the thread. A good O-ring costs little, can be changed in minutes, and makes the difference between a cache that lasts for years and one that has to be abandoned after the first rain.
In addition to replacement O-rings and silicone grease, we also offer complete hideout solutions where this sealing issue has already been considered, from film canisters to camouflaged PETling constructions. Those who prefer to print themselves will also find 3D print files with sophisticated groove design.
— Benedikt
Suitable products for a watertight hideout
You don't have to reinvent the wheel if you want to set up a weather-resistant cache. The 32×48 mm Geocaching film canister already comes with a robust, watertight cap and is suitable for smaller hideouts where being inconspicuous is key.
If you are looking for a slightly larger, decorative hideout, the Geocaching mini-birdhouse is a weather-resistant solution where you can optionally insert a film canister or a screw-top container and still benefit from the same O-ring logic. For hideouts in urban or rural outdoor areas, the bundle with two 60mm geocaching fence post caps is worthwhile; it unobtrusively camouflages PETlings and covers two hideouts at once.
Replacement O-rings in suitable sizes as well as silicone grease can be found in the equipment and accessories section. Measure the diameter of your current ring before ordering so that the replacement fits on the first try. Visit the shop directly and order the right parts for your next cache service.
Sources
The technical statements on groove design and material behavior are based on the Formlabs white paper on watertight 3D-printed enclosures, Prusa's practical tests on airtight printed models, and an O-ring construction example on Printables. Those who construct their own lids will find specific dimensions and print settings there as a starting point.
- 3D Printing Watertight Enclosures and Pressure Testing Results | Formlabs
- Watertight 3D printing part 2: Airtight closable models (Prusa)
- PETling watertight? | Geoclub
FAQ
Which material is best for the O-ring?
NBR or silicone rings are the most reliable choice because they remain elastic and absorb silicone grease well. Self-printed TPU rings only work as a makeshift solution with very soft material.
How often do I need to renew the silicone grease?
Best at every service, regardless of how the ring looks visually. Regular re-greasing prevents brittleness and keeps the seal functional for years.
Is hot glue sufficient as a thread seal?
No, hot glue becomes brittle over time and cracks with temperature fluctuations, which can allow moisture to penetrate via capillary action. An O-ring with silicone grease is the much more maintainable and durable solution.
Can 3D-printed lids be made watertight at all?
Yes, certainly with the right groove design and post-processing. Formlabs recommends a groove that is 15 to 20% shallower than the O-ring cross-section, while Prusa advises more perimeters and higher infill for FDM prints.
Where can I find suitable spare parts for my geocache hideout?
Cachewerk stocks film canisters, PETling hideouts, and suitable accessories such as replacement O-rings in the equipment and accessories section. You can find current prices directly on the respective product pages.