There are many types of treatments available, ranging from water based treatment through to fully pressurised tanalised treatments. Years ago one of the favourite treatments was crescote, often mixed with old engine oil. This did a sterling job protecting the timber from the harsh weather. This had a distinctive smell, quite strong, which tended to linger for quite a while.
Not ideal if you had treated the inside of a building you wanted to work in. Improved safety rules were introduced in 2003 as it had proved dangerous to use and from 30th June 2004 it is actually illegal to even store it or keep it in any form. A new substitute Crecote has been introduced for people who wish to get a similar effect.
Water based preservative are commonly used on the cheaper sheds, mainly serving the purpose of a base coat, for your second coat to be put on. The water is actually a carrier for the chemicals and helps the chemical to adhere and react to the wood thus protecting it to a certain degree. This can be either painted on, sprayed on or the shed panels can be dipped into a bath of this treatment.
Solvent and spirit based shed preservatives is the preferred and better method of putting treatment onto timber. The solvent again is a carrier for the chemicals and works far better than the water. The solvent or spirit helps greatly and gives greater protection to the timber. Again this can be painted on, sprayed on or dipped. Very few people dip their shed panels in this dues to the possibility of reactions to the operatives doing the dipping.
Oil based treatments and you are moving into the realms of 'Rolls Royce' types of treatment. The oil holds the chemicals and bond the chemicals into the wood. The oil soaks into the timber giving a certain amount of 'give' helping with the natural movement of your timber garden shed.
Tanalising, or pressure treating, is the very best you can do for your timber. The tanalising process consists of the timber being put into a sealed vat. And the Tanalith E fluid is forced into the wood under pressure. This ensure that the preservative goes a great deal further than just brushing or spraying. The result of this is that the timber can be guaranteed against rotting for a period of 15 years.
The treatment also protects against fungal and insect attacks giving your garden shed or garden building a long life. The appearance of the timber is quite attractive having a light natural green tint to the wood which can blend into the garden. However, you can add a colour treatment to the wood if you wish. This treatment is ideal for any building in an enclosed space where treating the building would prove difficult or where you like the idea of very low maintenance.
However whatever treatment you have it is important to keep an eye on it to ensure that you re-treat when necessary. The only extra treatment which the tanalised shed may need is a water repellent treatment, such as Thomsons WaterSeal. This would stop any dampness coming through if the building was subjected to torrential rain. (the timber is treated against rotting and is not a water repellent in its own right)
The water based treated shed should be re-treated within 4 weeks of delivery, ideally, and then every 2-3 years depending on the weather conditions. The same would apply with the spirit or solvent based treatments. With the oil based treatment after the building has its first coat you could get up to 4-5 years before anything needs doing again with the exception of any sides facing into the weather.
I do not recommend any of the water based treatments for the top coats and would only recommend spirit, solvent or oil base preservatives despite what the manufacturers may say. Water based treatments are fine for base coats or for fences, nothing else.
Some garden shed manufacturers claim to offer a 10 year guarantee, even with the water based treatment, however, like most things in life this is not as good as offer as it seems. There are strict conditions stating that if the shed is not treated EVERY year then the guarantee would be null and void. Clearly this means that the treatment they are applying only last for ONE year and the further protection is coming from the treatment YOU are having to apply every year. Clearly a WORTHLESS guarantee. DON'T be fooled by them. The best thing (or worse) is that you will be very lucky to find any details of the conditions on those sites.
With the benefit of my 39 years in the garden buildings business I hope that the above advice is useful for you to understand how to get the best out of your new garden shed or garden building. Over the years there have been many improvements and changes to the treatments available, some good and some bad, however, if you look after your shed it will last many many years.