By AMC Eagle Nest Member "Tennessee_Farmer".
First, a sagging headliner can not be repaired beyond just propping it up. The headliner material is cloth adhered to a foam backing. Once the foam disintegrates the cloth begins to sag. The headliner material and adhesives are readily available.
The job has to be done right or your new headliner will sag as well. Also, each time you remove the fiberglass ceiling molding that the headliner is attached to, it breaks up a little more.
Here is how I do it. Remove all the ceiling trim, visors, coat hangers, and ceiling light. The ceiling molding will now be free. Lift if off the hangers. Carefully remove it through the rear gate of a wagon or door of a sedan.
Remove the cloth and brush off all of the loose and gummy foam. The more you get off the better, but don’t brush to the point that you begin to destroy the ceiling molding. Try to do some repair to badly damaged areas, like where the visors are mounted and coat hanger areas. Cut some thick paper, like a manila folder, and attach it using a very generous application of white glue. Put some pressure on it and let it sit until very dry.
Next is the most important part. Your new ceiling liner will not adhere unless it is glued to a firm and dry surface. It will not adhere to the sticky residue that remains from brushing off the old disintegrated foam. Take a large bottle of white glue and mix it with water about one part glue to two parts water. Get it thin enough so you can apply it with a paint brush. Apply two to three coats, letting it dry between coats. Once it feels dry to the touch, you are ready to install the new liner.
Liner material is available on-line. A sedan takes 2 yards and a wagon 2 ½ (you usually need to buy an even 3.) I use www.automotiveinteriors.com where they sell the material by the yard and you don’t need to buy a kit. Stock colors are $13.50 a yard and other special order colors are $15.00. A sales person (Diane, 1-800-814-7533, ext 3) will help you match your color, if necessary, by sending her a sample.) You will need two cans of spray adhesive (16 -19 ounce cans.)
Have two people available when you glue the new headliner. Follow the instructions on the spray adhesive. It is a contact adhesive so you liberally spray both the back of the new head liner and the ceiling molding. Flip the headliner material over and carefully lower it to the molding, beginning the contact from the center and smoothing it out to the edges. Make certain that you smooth it to fit the contours, giving it lots of space to fit. Because the glue adheres on contact, you don’t have a second chance to pull it off and realign it. The headliner material is large enough so you have lots of overlap and you don’t have to be precise when lowering it on to the ceiling molding.
Trim off all excess material and cut holes for the interior light and centers of the visors. Use your old liner for size of cutouts, when necessary. Next, and very important, take an awl or ices pick and poke through from the back where the screws for the visors and the coat hangers should be. You don’t want to be probing to try to find where these belong once you reinstall your ceiling liner. Also, don't forget to tape your interior light wire to the roof so you are not probing for it later on.
The rest is just installing and putting things together. The hangers will hold the ceiling molding in place while you replace the trim. You will find the proper placement of the ceiling molding by lining up the holes you punched through for the visors with the screw holes in the windshield frame. Use your awl to find them.
Your header material may not look really even because of the pushing and compressing of the material you have done while installing. Give it a day and everything will even out and look beautiful.