Thack,
Well, yeah there is, and YEAH it will make it the finished job shmoother and shinier. The catch is that once you prep the pieces properly to remove all or as much of the silicones and treatment petroliums out of the pores of the vinyls/plastics, then you should wipe them down with a fast laquar wash thinner. This will do two things, 1. it will clean the surface and 2. also thin and dry out anything left after you have previously washed the pieces with an approppriate cleaner, like SEM SOAP.
Next, use a 3M scotch brite pad- Red, then Gray to scuff the surface to allow your first coat of resin to bite and sink into the pores of the surface. Use patience at this point and don't glob it on or use too much catalyst. Use a med. foam brush to get even coverage and low brush texture. Insure you flow over all your edges to decrease the risk of chipping after the job is done. after a couple to three even coats,and curing, you will be ready to finish sand.
Wet sand easy from 600-1200 grits to get a mirror smooth finish that will make you smile!Then do a flash coat of either base color or dye to get a good bite and fill the pores. The rest is all surface coat and clear. Remember to lap over all your edges as with the resin to lower the risk of the finsh chipping.
If you plan to dye/clear the pieces rather than paint/clear, I would recommend as a must using a premium grade of bonding clear like BullDog. This is applied to the surface just before your first dust coat of color. It is a wet application, so spray it on boldly and then just hang about a minute or two to allow the bonding agents to open the pores of the vinyl or plastic, and then begin your first color coat. Keep in mind the lighter the coats are applied and flashed, the better the cure. This is especially true for dyes because in reality you will only yield a mil soak-in depth on PVC vinyls, and just under that for extrusion molded plastics, like trim panels.
This is why it does make a difference if/when you "glaze" coat with Fiberglass Resins and fillers. The resin is a polyester copolymar and will bond well to most other polymars,which makes it suitable for our car audio creations. Once this is applied, we are dealing with a surface that is condusive to high solids coating like auto paint/clear, unlike the extrusion plastic surface underneath. But, be not mislead, Fiberglass is a plastic and will become mobile in varying degrees in it's structure once heated up, so keep this in mind if considering laying resin on a piece like a vinyl over foam dash. Just ask anyone you run across that has an aftermarket glass hood on their ride, they will tell you about the way fiberglass can move around when the sun leans on it for a while.
Obviouusly, the thicker the surface, the risk is delayed,but again its a plastic, and they all respond the same way to heat and cold. The heating and cooling abruptly can render some upsetting results on you, so consider this if you deal with extreme summer temps, and glassing a dash pad, or rear deck/trim that is in view of the sun, as I mentioned.
Hope this helps.
elrock-SS