I used SEAL-IT Wood Sealant when my log house was built in 1989. I varnished the exterior with one coat every two years using a garden sprayer. North Side of house and where the logs are in the shade, I have no need to re-varnish and it has not been re-varnished since 1991.
Pine shakes were used as shingles and some were showing signs of rot so I “spot” sprayed SEAL-IT Wood Sealant and can see excellent results.
Interior of house has one coat of SEAL-IT Wood Sealant and one coat Benjamin-Moore clear interior varnish. No change since day it was done.