GoodMark is probably using either 19 or 18 ga steel for their raw material so what it comes down to is how crisp and clean the finished product is when it leaves the press and how many times they whack it to get the shape they want and how new the dies are. The other point here is how accurately the inner braces and supports mimic the factory versions and if all the mounting holes for things like bumpers and gaskets and clips are where they ought to be and done in the correct diameter.
After that, there is almost always going to be fit and finish work necessary to get that consistent gap between adjacent panels and to get the surfaces smooth. The only way to know how much work will be needed is to remove the e-coating, or shoot over it if you wish, with a good epoxy or high build primer, shoot a second light contrast color "Check" coat over that, and start block sanding. The highs and lows will eventually show up and tell the tale. Along with this, there is the not so small matter of making new and old panels "talk" to each other so as to achieve that smooth professional look to the finish. Factory panels have their own share of issues starting with how crisp and neat the factory stamping was when it was punched out and how much abuse it has suffered since. Some of the factory dies got some serious use and were mostly used up by the time they were sold to the aftermarket. That high degree of wear can often account for why panels supposedly made using factory presses and dies seem to look so unfinished and sloppy.
For myself I have a Good Mark door panel in my shop just waiting to be unpackaged and fitted to an inner door skin for my 85 Monte SS. The preliminary inspection of the panel on delivery looked good but what I actually will get to work with have to wait until I initially fit the skin and get that first look at what results I get. Since the inner skin is, itself, anything but pristine and original, I will have to make sure that I get it as close to its original shape and dimensions as possible before I can even think about hanging that new skin.
With your fender, when you install it, you will have to shim it just as it was shimmed originally at the factory and start from there. Before you do that you will have to check your door hinges and make sure the door is hanging straight and aligned to the front edge of the quarter panel ahead of the rear wheel opening and gapped to get a consistent width at the door edge. If the hinge pins and bushings have worn to where the door drops or droops when you open it, then they need to be replaced and the door hung and aligned correctly before you can move on to the fender. Trying to set the fender to door gap to a door that drops or droops will drive you nuts and you could find that your door has just damaged the fender to door edge of your brand new fender when you try to open that door as a test. Essentially, the goal here is to see the panels, starting from the rear quarter panel, "Flow" into each other, one after the other, so that all the seams and panel breaks and other geographic points of interest in the metal align flawlessly from one panel to the next. And, just to complicate matters, that fender to door relationship also has to include the relationship between the fender and the hood; there is a gap there that has to be set as well, so you will be moving the fender in multiple directions to get things right. Are we having fun, yet??
This is not a show car exercise, although show builders do take this to the absolute extreme in the pursuit of perfection. This is just basic fit and finish. That said, it is not an exercise that can be omitted or skimped on because, once you shoot your color coat and get it out in the sun, any mis-alignment, dings, high spots, or tweaks, will be mercilessly exposed for the world to see.
Nick
