In order to achieve a 'real' finish one must start with structurally sound clean timber.
The oil penetrates and magnifies the appearance. As an example, grey timber will become greyer and timber with visible colour and character will come alive again.
As a good rule, only clean timber with products you would clean your hands with. There are many quick fix products available however they will invariably lead to a long term problem besides being harmful to the environment.
For defective stains and synthetic polymer coatings, the only way to prepare a suitable substrate, is by sanding.
Periodic cleaning down with a mild detergent is recommended as is the use of sugar soap prior to the application of a remedial coat of oil.
Grey and badly weathered timber can be restored with a very hot solution of sodium percarbonate. Due to the restrictions on selling this chemical in concentrated form, one can find milder but effective products on the supermarket shelves under the Napisan label etc.
All USA suppliers sell the product under the Oxygen Bleach label and claim that sodium percarbonate when mixed with water, breaks down into an environmentally friendly product which opens up the pores in the timber as it effectively cleans.
The amount of sodium percarbonate v water will depend on the degree of cleaning required and trials should be conducted to establish the optimum dosage. Care should be taken as the solution can leach out tannins, oils ( not always desired ) and protective chemicals.
It is not recommended to clean when the timber or ambient temperature is above 25° due to rapid drying.
For severe cases mix 1 cup of powder to 2 litres of very hot water in a broad bowl and then gently spread onto the surface with a soft broom or similar. The liquid will take a clear appearance turning to white and then grey/brown as the release of millions of oxygen ions takes effect. After about 10 minutes the solution should be scrubbed with a broom ( soft or stiff depending on what is most effective ) and then the surface hosed down with clean water. To minimize leaching, remove any liquid with a wiper blade or similar and allow to dry before oiling.
In order to establish a good starting point for oil, it may be necessary to repeat the cleaning operation.