Exterior wood stain, paint and varnish are the main choices for the protective coating the shed.
Have you ever paid attention to an older shed? You can spot them easily. With a bleached gray colour they appear to have been standing in that spot forever. So, what causes this weathering process? A combination of factors have a direct hand including bleaching from exposure to rainwater and the effects of UV light from the sun, exposure to pollution such as soot, or even mold or fungal growth on the wood itself. No matter what the culprit is, without proper preservation your shed will soon be that same dull gray colour.
Basic wood preservation starts with using pressure treated lumber. When lumber is said to be pressure treated it means that a wood preservative has been forced deep into the wood under high pressure usually in purpose made treatment plants. This then acts as a protective barrier and helps protect the outer few millimeters from exposure damage. Most wood sold for exterior use in the UK today has been treated to some degree in the way. Once you have seen to protecting your shed in this manner (normally by ensuring the external wood on the pre-made shed you buy has been pressure treated prior to manufacture or buying pressure treated wood for the external timber on a self build-shed) it is time to consider the protective outer coating.
The purpose of an outer coating on your shed is to help further protect the surface of the wood. A decent coating will help to protect the wood from harsh UV light which weakens wood and reduces its lifespan. It also helps to prevent damage to the wood caused by rapid changes in the internal moisture content of the wood (the coating also helps to shed rainwater). If not stopped these moisture changes can lead to splitting, warping and ultimately to rotting of the timber.
Once you are ready to apply a coating to your shed, there are three main types that you must consider. Exterior wood stain, paint and varnish are all popular choices.
Exterior wood stain is one of the most user friendly choices currently available. It is a breathable medium which allows the wood to 'breathe', and gain and lose moisture with the seasons. It is also available in a range of different colours which gives you the opportunity to find one to match the shed's location. Exterior wood stain is also very easy to maintain. When you are ready to touch up your shed all you have to do is to brush it well and reapply the stain - no extensive preparation required. You are then good to go for some time.
Many people choose paint because it is easy to buy and available in a wide variety of colours. Paint does have a few downfalls however. If the wood that is receiving the paint is not properly prepared (usually through priming and/or application of a moisture seal) then moisture may become trapped beneath the paint which will cause it to bubble and peel. Cracks or missed spots in the paint job may also allow moisture into the wood which will cause damage over time. Painted surfaces usually require repainting (requiring extensive sanding, filling and preparation) every five years or so.
Varnish is very similar to paint, however, it contains no pigment and so is 'see through' revealing the underlying beauty of the wood....initially. If/when water gets behind the varnish it stains the wood which can be very unsightly, also any mildew or fungal growth that occurs can be seen beneath the coating. Varnish has a long drying time which can allow dirt, dust, and other contaminants to settle on the surface and become trapped which can damage the finish. It also can be time consuming and tricky to apply as it must be stirred slowly to keep from forming air bubbles.
No matter which type of coating you choose there is one more decision to make. Whether to hand paint or to spray coat is a question that many people ask. The answer is really up to you, the size of project and the equipment you have available. Hand painting is especially good for taking care of end grain as well as for getting the liquid into surface details. If you are going to be painting a large surface area then spraying will better suit your needs for the biggest part of the project. It is very good for covering large areas in a short time. A good choice is to use a combination of the two methods: spraying for the large areas of the project then go back and hand brush in the details and end-grain.
Painting your shed is a very important part of ensuring its longevity. No matter if you use paint, varnish, or exterior wood stain, or whether you spray or brush it on, with the right care and upkeep you will be able to enjoy your shed for quite some time.