Need Some Mitering Help

[Guest]

Yes. It will affect the miter angle and depending on the angles, the length would be affected. Actually, since it's wood, the opposite faces of the post probably aren't the same. The post is probably not square (rectangular) but it's also probably not a parallelogram either. Thus the need to fit each piece separately.

You might also consider the Tage Frid method of fitting joints. Beat the corners together with a hammer.

[mike4244]

I've been doing this well over 50 years. First thing a miter joint on trim if outdoors will probably open up anyhow. Use a PVC product instead of wood and PVC glue for the miter joints. Check the post for square. If the post is fairly square, less than 1/32 gap at a corner then use the following method. Cut  4 pieces 1 or 2 inches over size if you are working with a long board to start with. Set your miter saw to 45-1/2° and make the first cut. Hold this piece to the post with the short bevel flush or a red hair past the post, then mark the other end ,top and bottom. Square the bottom mark up  and compare it to the top. Use the longest measurement for the cut line. Swing the saw the other way to 45-1/2° and make your cut.  Tack this in place. Repeat for the other three sides. When you are satisfied with these joints then remove and glue the miters up before nailing tight.
If you use PVC glue and PVC trim boards then hold each joint together for a minute or so. I usually wait another 10 minutes before nailing tight.
I often glue up 3 sides, wait a bit before nailing then add the last piece.
If you run into areas that are not square corners then measure the angle with a bevel square ( also called a sliding T- bevel). Check the angle, divide in half and add 1/2° to the angle to be cut.
The added 1/2° to each side of any miter insures the front will be tight. Theorectically the back should be open 1°  but this seems to be hardly noticed, probably because you see the thickness of the board about 3/4" instead of the entire height of the trim if the front was open.
I hope I helped,
mike

[Steve N]

Thinking of inside corners needs to be different than outside ones. Inside corners are usually 45* or LESS, while outside corners are often 45* or MORE.

That is the first thing to think of, the next is you need to have an accurate PROTRACTOR to show you exactly what the angle is, because over 45* is going to be what is called an OBTUSE angle.

Gonna let Jane (Leah) splain it to you, she's a hoot. Report back if doing it like Jane shows you doesn't work.