I've had a sagging headliner in my S70 for almost a year and I've finally gotten to the point where I can't take it anymore. After thinking it through a bit I came up with the following steps to repair it without removing the whole headliner. Most of the write ups I've seen are for wagons in which removal seems much easier than a sedan since the headliner can be pulled straight out the back.
1. Find some pieces of thick cardboard roughly as long as the headliner is wide (trim to trim). 1 piece needs to be as wide as the depth of the section just behind the sunroof. The other needs to be about half that width to fit the rear most section of the headliner. When you feel around you'll clearly notice the difference in the sections of roof I'm talking about. Basically find 2 separate pieces to cover those 2 sections effectively.
2. Find in total 6 devices to prop those cardboard sections up against the headliner in order to apply even pressure against the material. I found 3 wiffle ball bats to work perfectly on the front section. I cobbled together some other stuff in the garage to fit the back section. if you have some adjustable rods that would work perfectly. I used an adjustable emergency snow shovel, an adjustable microfiber car wash wand and an auto snow brush/scraper thing. Feel free to cut your own 2x4s or whatever to fit.
put all that stuff aside now. Not too far though because you will need them soon. https://flic.kr/p/z4WXWG https://flic.kr/p/z5Y1Mg
3. Unclip the sunroof trim ring, mostly just the rear edge giving you access to the large section of headliner to rear of the sunroof where it sags. Cover that section of trim ring with blue painters tape to protect it from the glue. https://flic.kr/p/y8Wr8o
4. Gently pull the material down if any of it is still glued up. this will give you lots of access to the space between the headliner material and the actual headliner .
5. Using gloves and a mask/respirator with plenty of fresh air, slide your can of 3M adhesive spray or headliner spray sideways into the gap in the headliner and get a good coat on the back section of the headliner. following the directions on the can wait a minute or 2 for the glue to get tacky and then slide your cardboard and supports up on the section to begin the adhesion process (on that section only!). https://flic.kr/p/z4WYpA
6. Come back to the open section of headliner material and now spray that front section with a nice even coat. Be sure to get some glue close to the front edge of the material (without overspraying all over the inside of your car). Again give it a minute or 2 to get tacky and slide your cardboard and supports (wiffle ball bats
) up against that section of headliner and let it all sit there for a while. Technically I think it's probably all adhered within 10 minutes or so but I left it on there for at least 30 for insurance. Then just remove your cardboard and supports and carefully reinstall the sunroof trim ring and viola! Like brandy new!
Some considerations: try to find the smoothest pieces of cardboard you can so they don't "dent" the headliner material. If your headliner needs cleaning I would suggest doing that first especially if you may be steam cleaning it as that might compromise the adhesion of the glue. Be sure to test your cardboard and supports thoroughly before continuing the process. Firm even pressure is key. I'm sure temperature and humidity could have some effect, use some common sense here.
The whole project took me about 40 minutes (not including drying time). 30 minutes of that was spent finding/cutting the right size cardboard for the rear section and finding the right size supports all around.
Good Luck
