Originally Posted by Longarm
When cutting triangles from a square if you cut once from corner to corner the long side is a bias. If you cut twice from corner to corner the 2 inside cuts are bias and the outside is a straight grain. When I teach I tell my students to put a pin or some kind of mark on the straight of grain edge so they know which is the straight of grain when putting pieces together.
I know this won't help current problem but it may save frustration in the future.
Longarm
Now that is a smart little tip! I'm going to keep a chalk pencil at my cutting station just for this. Thank you!
I know one way to avoid wavey borders is to never just slap the fabric on and start sewing. It's important to measure through the
center of the quilt to get an accurate measure of the true size of the top. Measure the width through the center and cut out the borders to match these measurements exactly. Then pin the border to the quilt. Pinning may not seem important but it keeps the feed dogs from shifting the fabric, which is a great way to end up with puckers or a wave. If you first pin the edges, then match up the centers and pin that, then you can begin to pin the rest of the border to the quilt top. This way if you need to ease or stretch the fabric at all, you'll be able to do it across the entire quilt evenly, again so that no one area puckers or waves. Once your side or top and bottom borders are on, simple measure through the quilt top again for the correct measurement and cut out the last two borders to fit. Pin, sew, and enjoy borders that don't wave all the time.
Hope that makes sense and helps someone.