When I took my car to the blaster, I got the car back with all of the factory coating on the inner sides (in the trunk) of the quarter panels blasted off from just inward of the rear of the wheelhouses back to the taillight end of the quarter. However, I didn't notice at the time that they didn't clean it up far enough in past the wheelhouses to where you couldn't see any remnants of the old coating. So, the old coating is visible when looking in the trunk. When I had the car painted, I asked the painter to reapply coating to the areas that were stripped before he painted the car so that it would blend with the old stuff that wasn't removed. He told me he was going to use Duplicolor truck bed coating and assured me it would blend with the leftover old coating and I wouldn't be able to see a seam. Well, the stuff he sprayed on was nothing like the factory stuff, much thinner, and as a result the seam where the two meet is very visible and looks like crap. So what I would like to know is, does anyone sell a factory type thicker coating that I could apply over the existing painted coating he put on and then paint over to match? Something I could brush on would be ideal but if I have to spray it that would be O.K. too.

So far, the thickest stuff I've seen is RestoRick's. Takes some practice to get it to look factory, and it's expensive.
If you start experimenting with rattle-can undercoating, check to be sure it's paintable.
I'm sure others will reply, as this is a problem that still plagues us.

RestoRicks is very good undercoating.Don't try to get it done in one coat, it will take a few coats to get the thickness and the texture correct.

This is correct. Most of today's undercoating goes on much thinner than the old/stock undercoating (but the old/stock may very well have contained asbestos... so we don't want that). Anyway, you can get weird imperfections if you spay on a modern undercoating too thick (drips, solvent pop, crazing, cratering, etc). The best way to "tie together" multiple coats of undercoating is to spray a coat, let it almost dry, re-coat & repeat until you get the thickness you want. That way you avoid most/all of the problems & the lower layer can partially mix with the upper layer.

You also could completely remove the old/stock stuff first so that at least everything would look the same, if not original.

Also, with the new stuff don't fully depress the nozzle. It will spray thick and less consistently. Short of getting a Schultz gun with all the goodies that is possibly the best rattle can solution.

I also found that using lower air pressure gave better looking results than higher pressure,a little lower than Ricks tech sheet recommended.

RestoRicks is the only way to go! If done right it's virtually impossible to tell from the original.

I've used Lord Fusor sprayable seam sealer with great success, and it is also paintable.