Borders and Miters
To ensure a proper seam, seal all seams prior to seaming, as shown in Photo 6. Construct border seams up to miters, as shown in Photo 7, which shows a common method of constructing hot melt seams up to the mitered corner. Mark the bisecting points (Photo 7) Cut the first angle (Photo 8). Next, as shown in Photo 9, overlap the angle cut over the uncut border and mark by carefully cutting a small slit at each end of the miter; then cut the second angle (Photo 10).
Photo 11 shows a method that I see quite often: the seam tape that is placed under the mitered corner is cut to fit to the two seam tapes.
Here is a method that I have found that works well for me. As shown in Photo 12, cut a piece of seam tape to fit the mitered corner. Cut the portion of paper that does not have any thermoplastic. Trace around the seam tape and transfer the marks on to the other two seam tapes. Gently score the two seam tapes along the trace line and remove the seam tape. If you have difficulty removing the tape, clean the bottom of your iron on a scrap piece of carpet and slightly heat the tape from the back and remove. Use one end of the iron to heat the two areas to be removed. Do not place the entire iron on the backing as it may distort the backing and this will be visible from the face side of the carpet.
You must remove the paper on the edges or else you will have a gap on each side of the seam that will create a weak point. The circle is where the intersecting points are located; this is a structurally sound seam with no overlap of seam tape (Photo 13). Compare this photo with Photos 1 and 2, and you should see a big difference.
We were able to save the Wilton job, but removing all of the seam tape was required. Hand sewing the fills and borders was required for a proper installation.