Hi John,
I can offer some good advice to help you with your problem. So ----------
DIAGNOSING THE PROBLEM.
To the best of my knowledge, these rigid trim panels were made from a gel coat type material with a fake vinyl grain impregnated into the material. Looking at your photo, the good news for you is that the condition of the overall finish of your panel appears to be very good and original. With age, and sun rot, a lot of 123 Mustang trim panels start surface breaking down and rotting away. This rot makes the surface go soft, chalky, and powdery, and crumbles away to the touch.
In your case, given that the original surface is in good condition,and the fake grain intact is a real plus. However from what you say, the previous owner has used a hot melt glue to attach speakers to the trim panels. That's where the damage gets done. Trying to remove that glue residue successfully without damaging the substrate is a real challenge. It must be also understood that any sanding of the original fake vinyl finish would start to remove or wear down and thus change the look of that vinyl finish, creating flat spots. Using certain stringent solvents to wipe over or clean the original surface, can also have this similar effect.
Looking at your photo, there still appears to be traces of the textured fake vinyl finish on the damaged area, but i can't be sure of that. In any event, you have sanded down that area, and so will have changed the original look of the finish. So, what can be done to try and fix the problem?
FIXING THE PROBLEM.
There are two basic ways you can go about repairing your trim panels. These revolve around your approach to the repairs. The first approach is DIY, and you will be using rattle cans only to carry out the repairs. The second approach is more professional, where you will be using auto spray paint shot from a spray gun and a compressor. Either way, my advice would be to go with using acrylic lacquer paint instead of 2pack paint in your case, mainly because it's more user friendly for the home guys to work with.
To make a successful and good looking repair, you will need to create a simulated vinyl textured finish to try at best to match the original fake vinyl finish on the trim panel. Looking at the first approach, using a rattle can product like SEM's textured spray finish would be a good way to go. The procedure then, would be to prep up the panel first by giving the entire panel a very light dry scuff over with 1200 wet & dry sandpaper, then use a wax and grease remover to clean down the entire panel. (I assume you don't want to disturb or remove the spray paint you have just put on yourself, or the older paint the previous owner put on). Apply the texture coating paint to the damaged area, and as you are spraying the paint on, back blend the paint progressively into all surrounding areas adjacenct to the small repair area.This blending technique helps disguise the repair area. You can also give this paint a scuff down between dried coats if you don't like the look of the finish as you are applying it.When you are happy with the look of all that, and the textured paint is dry enough, spray the whole trim area with a lacquer paint of your choosing and color and gloss level. This procedure should give you a decent finished result.
The second approach involves using a spray lacquer primer or putty product from the paint can, reducing it with lacquer thinner about 5 to 15%, and then shoot it through a putty gun or a gravity fed color gun (non HVLP) with a 2 or 3 ml setup. The application technique involves dropping your spray pressure down to around 5 to 10 psi at the gun head, and then apply light spatter coats to the repair area, and blending out to the surrounding areas as i said before. Again, you can scuff down the applied and dried texture coats to get the finish you like, and then drop another few coats on top of that. You get to control the look you want to end up with. When you're happy with that, move in and spray the entire trim panel with an Acrylic lacquer and color of your choice. Don't forget about choosing a gloss level that suits you as well. Also, Sem make their texture coat for a spray gun application as well. You could go that way if you wanted as an alternative to using the lacquer spray putty. I would go for a low satin finish myself, but matt is OK as well. This technique gives a great finished look as well.
See my references below to help you out. -
https://www.semproducts.com/refinish-flexible-coatings/texture-coating
https://www.google.com.au/search?q=sem+texture+paint&tbm=isch&imgil=t2H-uSeSyWamLM%253A%253BPXGrFlkf1SsbUM%253Bhttp%25253A%25252F%25252Fwww.f150online.com%25252Fforums%25252Flightning%25252F218873-sem-texture-coating-3985-a.html&source=iu&pf=m&fir=t2H-uSeSyWamLM%253A%252CPXGrFlkf1SsbUM%252C_&usg=__3oU2vM55is3fRhUDYAhrPmqCLUY%3D&biw=1920&bih=943&ved=0ahUKEwiu2MGRo_HLAhWDe6YKHarmDRcQyjcIJw&ei=JWwAV-6xI4P3mQWqzbe4AQ#imgrc=_
Hope all that helps,
Greg.