A guitar case has a hard outer shell and an inner support frame, which is most often made of foam. The liner material is laid directly over the foam support and affixed with a clear adhesive. When replacing the original liner, select new material that is nonabrasive. Some people choose to replace old liners with expensive materials such as velvet or silk. Carpeting, especially plush and shag varieties, also makes a suitable replacement liner for a guitar case.
Remove the old liner. The easiest way to remove the old liner is to locate a loose edge and begin to peel it toward you. If the material is worn or damaged, you should be able to locate a loose spot fairly easily. Turn the case so the loose area is as far away from you as possible. Use slow, steady movements to peel the liner from the foam core, being careful not to damage the foam as you work.
Measure the length and width of the inside of the guitar case. Note each measurement and add 4 inches all of the way around.
Locate the centre of the case. Use a light coloured piece of chalk to mark a line down the centre, from top to bottom.
Lay out the material and measure the appropriate length and width (plus the additional 4-inch margin.) Use the chalk to mark the material for cutting. Use scissors to cut the replacement material along the lines previously marked.
If you are using carpet as a replacement liner, turn the carpet upside down on the work surface. Mark your measurements on the back of the carpet using the chalk. Cut along the previously marked lines with a box cutter.
Fold the material in half lengthwise. Place the fold along the centre line. Unfold, keeping the centre in place. Ensure that the material fits properly. There should be enough of the replacement material to cover the entire inside of the case. There should also be a margin around the edges that can be tucked inside the edge all of the way around the case. If the material fits properly, take it out of the case and refold as before. If it does not fit, remeasure and cut to size.
Spray the inside of the case with spray adhesive. Working quickly, drape the material into the case, lining up the fold with the centre line on the bottom of the case. Unfold and drape lightly on the other side.
Work from the centre outward. Press the material flat and smooth out all wrinkles. Use a putty knife to tuck in the edges all the way around the inside of the case.
Treat the lid of the guitar case in the same way as the body.