Linseed oil should be applied to your cedar wood regularly and often. You can also apply it in between coats of stain. You may have to apply more than one coat of linseed oil depending on the extent of the problem. Linseed oil will take time before you see the results. Your deck or fence will need to sit for at least three months before you see any significant improvement in its appearance and performance. When applying linseed oil, choose a quality product that is formulated with natural ingredients so that they won't leach into your cedar wood over time. A popular brand is "Cedar Shield."