To prevent water, bugs and air seepage, your log home must be caulked. Any openings, windows, or doors need to be caulked as well as pass ends (outside corners), butt ends, inside corners and certain log checks (cracks in the logs).
Ideally, the caulk will be the same color as the finished stain coat. Backer rod is used to fill large gaps before the caulk is applied, not just in checks, but between D-logs and window/door trim. It is very important that the right amount of caulk is used and that it is applied correctly. Too much caulk will sag. The caulk must also be neatly tooled into place.
The material that is applied between the logs on some types of log homes is called chinking. It not only gives your home that old fashioned look, but also helps seal it against water, bugs and air filtration. Chinking comes in many different colors and can also be painted (as shown below).
Borate treatments are used to help prevent damage from rot and wood boring insects. Usually, the borate solution is applied to the bare wood just after the log home has been stacked and before any stain is applied.