When i was a kid i used to white wash the milk barn with slacked lime and water. That stuff lasted a long time,the last time i did it was about twenty five years ago and it still looks pretty good.
I don't remember which one of the old timers in my family told me but they would put their sour milk in a barrel with some pieces of rusty iron to make red barn paint and they may have added some lime to it before painting with it.

In days of old, there weren't too many Kelly Moore, Benjamin Moore, Sherwin Williams or other paint stores around. People had limited choices for protecting their wood projects and adding a bit of color to their lives. Still, the choices they made worked well. Among those choices was the use of milk paint.

Milk paint can be described as having an antique-looking finish. It forms a hard matte surface and its grainy look is distinctive. For these reasons, it is yet popular for reproducing period furniture and artist and craftsment use if for decorative effects.

Milk paint can be tinted. You can add lime to act as an insecticide and fungicide to protect from insect and fungal infestations. It dries quickly. It is biodegradable, non-toxic, and odor-free, once dry (it doesn't smell like sour milk). It is made from milk casein, clay fillers, lime, and earth pigments for color.

You can apply milk paint directly to raw wood, without the need for a primer or sealer, since a naturally forming resin called calcium caseinate allows it to bond well to the wood.

You can buy it ready-to-mix at http://www.realmilkpaint.com or pre-mixed at Rockler. Alternatively, you can make your own milk paint:

1 quart skim milk (room temperature)
1 ounce of hydrated lime (not quick lime) by weight
1 to 2 1/2 pounds of chalk may also be added as a filler.

Stir enough skim milk into hydrated lime to make a cream. Add balance of skim milk. Now add sufficient amount of limeproof powder pigment to create the desired color and consistency. Stir well for a few minutes before using and during use. Extra paint may be kept for several days in the refrigerator, until the milk sours.

Now that you've built it, how do you keep it in good shape? In hard times, maintenance materials may be hard to come by and you really can’t afford to replace a project, for the want of time or money. Wood has a limited life span under adverse conditions, if not protected. Even treated wood decays rapidly, if allowed to dry out. As a result, it shrinks, causing splitting and cracking that exposes the center of the wood, where water gets in and freezes, expanding he crack and where it begins to rot.

If you've constructed your project from iron or metal your choices for protection aren't complicated. However, if you've constructed your project from wood, you have many more choices to make.

For metal, you just need to apply a good quality paint. Of course, there are things you have to do to prep for the paint. This may include removing oil and priming the metal. After that, it's walk in the park (things like silicone contamination aside, which may call for shellac to seal the surface). Just a good sealing paint, which will not allow air and moisture to penetrate its surface, will do. Any paint designed for metal surfaces should do, since they seal (except for miss-mixes that are marked down, the cheaper the paint, the less quality the paint). Be advised, latex paints are porous and will allow air and moisture to pass. After you have a good initial coat, you can use latex, but you may be better off staying with a paint intended for metal surfaces.

Dealing with wood and having it survive long term can be more of a challenge. Do you want to paint the surface, or do you want to leave it natural? As with metal, painting requires preparation, like priming. But maybe painting is not be practical due to time constraints, or some other reason. If so and you still want to protect the wood from the elements, you can consider oiling the wood. This doesn't always have the advantages of ultraviolet protection, but it can extend years, in terms of decades, to the life of your project. Even without the ultraviolet protection, the damaged outer layer of the wood provides some protection to the under layers.

Oils are old technology, but they are my favorite choice for exterior surfaces like decks and so forth, if only as an initial treatment. Among the reasons I love oil is it is easy to put on. You can use a brush, a roller, a mop, an airless, an HVLP system or any means my imagination comes up with to get the oil on the project. I often use a pump-up garden sprayer, rather than my commercial spray equipment, for a small fence. I can do a one hundred foot fence in about an hour. To start, I first thin the oil, or it is too thick to spray. I start at 25% paint thinner, turpentine, mineral spirits or naphtha, whichever is cheaper (the only things that really changes between the different thinners is flash point) and I add more as the weather cools. It is not rocket science and you can go by feel. The excess thinner will just evaporate off.

Oils have many wonderful characteristics and are still used in paints (e.g., oil based paints use boiled linseed oil). You have two choices - hardening or non-hardening oil. As their names imply, these oils either harden over time, or they don't. Non-hardening oils are things like motor oil and mineral oil. It also includes "lemon oil" used on furniture, which is really nothing more than mineral oil with lemon scent added and baby oil, which has no business on a baby, by the way.

BREADBOARDS AND BUTCHER BLOCKS

Mineral oil is what you should use on your breadboard or butcher block. Never use olive oil or cooking oil. They will go rancid over time, even if you cannot tell by smell. If you insist on a hardening oil, you can use raw Tung oil or Walnut salad oil. Read the label though. Many products promoted as tung oil are actually thinned tung oil, rather than pure raw tung oil, so shouldn’t really be used on food surfaces.


For outdoor projects that are not going to be painted, I favor non-hardening oil, which can be obtained from various sources. If you consider using used motor oil, remember that it should not be used where it would frequently come in contact with skin, since it has a lot of contaminates, to include metals and such. Another source of oil is Chevron, which offers Shingle Oil. You will be told not to thin it, but I always do.

Regardless of the source of oil, I thin it. I want deep penetration. Remember, the solvents will evaporate off, leaving the oils behind. When treating wood, I apply my first coat. It usually happens that, by the time I get to the other end, the oil I first applied has already soaked in. As such, I immediately start applying more and continue to do so until it does not quickly soak in.

Oil, given a chance, will saturate wood. Wood saturated in oil is not going to allow water in, except on the surface where the oil has been weathered or evaporated away. Too, well oiled wood is not going to rot. A few weeks after a generous application, you will notice small cracks and spits seem to have disappeared, since the wood has swollen from the oil, just like it does in the rain. When you think you need more oil, just add it. Unless your wood is buried in moss or a seal coat, the new coat will soak in.

A thing to remember about oil applications is they are cumulative. Deep penetration is critical and that means thinning and generous applications. The first application may seem to disappear quickly under the onslaught of summer. A second coat will appear to last a little longer. A third will be notable a few years later. If in doubt, try these applications on a cedar fence near a neighbor’s untreated fence. If you are aggressive at the start of your treatment program, the results will protect your wood years after the neighbors have replaced theirs and, in the mean while, the difference between the two fences will be obvious.

CEDAR SHAKES AND SHINGLES
If you were tending the cedar shakes or shingles on your roof or siding, you would use a non-hardening oil. The oil will not make roofs look new, but they will look better and they, certainly, will last years longer. Too, because of the oil and assuming you were generous in your application, the cedar, or any other wood, remains resilient. Normally, walking on a cedar roof in the dead of summer, when the shakes or shingles would be their driest, would cause them to break. If saturated with oil, they will merely flex. Too, the oils expand and contract with the shakes. As such, there is no problem with the seal coat cracking open and allowing water in.

Never apply sealers to cedar roofs. An example of the poor results of this can be seen in roofs that magically become new overnight, by a mere application of one of the various products available for such purposes. Unfortunately, these are highly pigmented products and they look terrible in as little as a year. But the damage is done the day the orange pigmented product is applied. Oil can probably penetrate the cracks of the now ugly orangish product and provide protection, but it will still be ugly.

OLD WINDOWS AND SUCH
If I were treating an old wooden window sill before repainting, I would switch to boiled linseed oil. Linseed oil is a hardening oil. It’s made from flax seed, but has heavy metals added to speed the drying time. Again, I would thin, which also speeds drying, and allow it to penetrate the wood. Next, I would allow it to dry for several days before priming and painting. The advantage of this is, even if the paint cracks, water will be repelled by the linseed impregnated wood underneath. For your old window glazing, you would use straight linseed oil on the wood before applying the glazing. This stops the wood from drawing all the oil out of the glazing.

Other hardening oils include Tung oil and walnut salad oil. Most tung oil is thinned and just made with raw tung oil, so read labels. Walnut salad oil tends not to darken wood like boiled linseed oil and it doesn’t have the smell associated with boiled linseed oil.

This oil information can be applied to any wood, including decks. However, you may have to wash away excess with a good detergent.

CRITTERS
If I thought I had problems with critters nibbling at my house, I would consider dissolving copious amounts of Borax (Twenty Mule Team or other) into warm water and spraying it on my woodwork. The problems with this is rains will wash it away in time. However, under a house or another protected area, that is not a problem. If you do apply it in a less protected area, you could allow it to dry, then oil the surface to minimize the leeching away of the borax.