Recently on http://www.thingiverse.com/ there was a contest called "Make it Float" which made me question whether a 3d printed part will naturally have the capacity to keep water from flowing in through the delicately laid lines that make up the piece. After a little research and checking out some of the different prints submitted to the contest I concluded that indeed a part can be printed in such a way that it repels water and floats.However, parts printed with "standard" settings and with an optimized print time in mind do not commonly have this ability. The plastic naturally floats in any water but it is more like an ice cube than a foam board (floats IN the water not ON the water). This brought to mind an issue which needed solving... how to make any print water proof, even when gaps between lines occur.
The world of 3d printing is expanding at an incredible rate. People are printing houses, cars, bicycles, and boats. With this influx in prototype and production, prints will undergo all sorts of different environments and stresses. The environment that this instructable deals with is liquid, and how it effects your print. I will show a way to apply a coating on your print to prevent liquid from entering your infill, or deteriorate your outside.The coating of choice will be WAX because it is readily available to most people, but there are many other types of coatings that will do the same thing, and possibly do them better.
Coating material is commonplace in most every industry, Carbide tooling is often coated in a TiAlN (Titanium aluminium nitride) to prevent wear while cutting, allen wrenches and socket heads are often coated in a black oxide to prevent rust and wear, and even couches and beds are coated in a flame resistant resin in case of fire. 3d prints currently use coatings for many different reasons also, to smooth edges, to increase toughness, and to block liquids from entering. So WHEN your print is in need of any of these attributes look to coatings for help.
Each 3d printed part is different so discovering WHERE is unique to the instance. A few rules I use are as follows: does my print interact with liquid on a regular basis, if yes then WHERE does it interact, apply the wax to that specific spot. If you are not sure WHERE it interacts, just apply wax everywhere and you will be sure to get it. An example would be a 3d printed boat (like this one from Thingiverse http://www.thingiverse.com/thing:763622 a great print to test the capability of your printer also) most boats only need wax applied to the bottom of them, and a small amount of the lower portion of the sides. Use your discretion as to the best place to apply wax and if all else fails, just put it everywhere!!!
I have already answered this question in the WHEN and WHERE sections but 3s are the best so I will reiterate it here. Wax on a print will prevent water (or other liquids) from breaking into the print because wax creates a water tight barrier. This is WHY you should use wax to coat your objects.