Your log cabin is untreated. It is therefore vital to treat your log cabin as soon as possible after assembly with a high-quality wood preserver. We would recommend applying several coats and try get a good thick layer to the bottom few logs on each side and around all log joints.
If you wish to treat your log cabin with a paint, you may want to use a clear preserver as well as the paint. Speak to the paint supplier / manufacturer for more details.
You then need to re-treat the garden building regularly (typically once a year) to ensure the logs and thoroughly protected. We would also recommend treating the inside of your log cabin as well as the outside.