Window trim on flat log walls
I'm finally tackling the inside window trim on my house. I've got 2 situations that I need guidance on. One is on the walls that have flat, laminated logs - see the first pic. I plan on using 3/4" jams and flat, 1/2" X 2 1/2" trim butted, with an apron and 3/4" stool. The logs end at the rough opening of the window and have OSB lining the edge. The edge of the OSB is straight but the logs are not even with each other and therefore not with the OSB. In some areas, the OSB is proud of the logs, in others the OSB is inset from the logs. So I have a very uneven surface for the trim. I can remove the OSB that sticks out from the logs, but I still have the unevenness of the logs to deal with. The width of the jambs is nominally 1 3/4" but vary maybe by 3/16" or more around the window. Is the best thing I can do is pick the widest jamb width number from all three sides and make the header jamb and side jambs equal to that? Then just accept the gaps between trim and logs that will occur on the outside of the trim? See the third pic.
The second situation is similar to above but is on a 2 X 6 wall that has siding mimicking the log design and does not have any OSB material. Although the siding pieces are relatively flush, they still create variation in the jamb dimension, nominally 4 5/8" but varying by 1/2" within one window - see second pic. Here, I'm thinking I have to put the trim on first and then hold up a wide jam and scribe to the trim piece. However, I don't think I can cut that scribe line very accurately to get the jam-to-trim joint at the reveal to look very good. All this is being done with stained cedar lumber so caulking to fix anomalies is out although I am using a solid stain that might cover wood putty.
Are there any other options that will make it a more professional looking job?
Thanks for your replies!