The basic idea can be used with all fiberglass parts, as an example I will use a hood. You need to find the point on the edges of the hood that are the most "too high." Let's say that you have a decent fit but along the top of the fender it is too high at the front and too low everywhere else back to the rear where it is high again. You can’t “adjust” the middle of the low spot up, at least not with the hinges or latch. Align the hood as best you can with the high spots all being just above the adjacent panel. You want to sand the high spots down until you are happy that you have cut it as much as possible without jeopardizing the integrity by thinning it too much. What you are after is a “compromise” from the highest spots to the lowest spots. If you can cut the high spots enough to make the thing fit, great. If you can’t cut that much off, then you need to get it as close as you can so you don’t have to raise the low spots up as much. If the fiberglass is thick enough you can grind it down through the gel coat, don’t worry about it. A skim coat of polyester putty will fill pin holes in the glass. Then if you need to, align the hood again so that highest point is just shy of being perfect to the fender, just a tad low. Now shave the sides of the hood if needed until you have best gap. You do the same thing as the top. Find the widest point and shave it until you are happy that you have went as far as you can without cutting off too much. On some parts this may be an area you can cut a ton off without a problem. Other parts may not give you that option. If it had a lip that folded down, you don’t want to sand it off. You can “thin” the lip, but that is all. If you really needed to cut a lot off, you can of course re-make the lip. But this is not usually needed, the part wasn’t made THAT bad, or at least we hope not. Do what ever it takes to get the panel as close to fitting as possible. If that means grinding mounting points thin, shimming, what ever it takes, do it now. This may include (and most likely, will) aligning the adjacent panels. Like any alignment of panels, it is not one panel, you are looking for fit between multiple panels so it takes alignment of all panels involved to make them fit well together. Now, I have had fiberglass parts that were SO bad you couldn’t grind the high spots down enough without grinding through. In this case you need to build up the thickness of the part at these points so you can grind the top down enough. All it takes is to turn the part over on it’s back. Grind the area so that your resin and mat will stick and apply some there. When it cures, put the part back on and grind the high spot down as needed. If you need to grind all the way through the original glass to make it fit, so what, you now have made a “new” point where you can stop grinding by making the glass thicker. I have ground off mounting points to bring a part in closer and then made a new mount. Some of these aftermarket fiberglass parts are not very good, but if it is all that is available, you do what you have to do. By the way, there is “Mat” and there is “Cloth”. Mat is fiberglass strands laying together in a random non-pattern and is usually thicker. Cloth is neat criss cross fiberglass strands. Mat will FILL much more. Cloth is used for structural support where strength is important. But I have to say, Mat more closely resembles how most fiberglass parts are made anyway, even structural points in them. Most all parts are made with a “chopper gun”. This is a “spray gun” that shoots resin and a “Spiderman” sort of web of fiberglass strand being chopped up as it comes out of the gun. It is sort of the “MIG welder” of fiberglass. There is a spool of fiberglass strand that it pulls from and then chops it up as it mixes with the resin and hits the surface of the mold to create the part. Mat is basically the same thing, the resin is just not there yet. On these points you need to thicken, the mat will do a fine job leaving all the structural integrity. How to use this mat and resin will have to be another “Basics”. You will probably have a few areas that the gap is too big for your liking. You want to sand that edge with 80 grit so the gel coat is good and roughed up. Now, sand the hood on top where it is low, sand REAL good out into the hood with 80 or even 40. I don't like to go out too far with the 36 because you are going to be feathering it out and you don't want to have those scratches to deal with. But if you know that it will be a way from the feathering area, 40 is better. You should have the hood fitting as good as you can with the highest points on the top just shy of the top of the fenders and the edges so that the narrowest gap is "livable." The edges need to be sanded well and the top is ready for filler in the low areas and in from the edge maybe as much as a foot towards the center of the hood. Now before you read on, think about this. The hood is fiberglass, right? It is polyester resin with fiberglass strands that give the resin strength, that is ALL it is. The gel coat is simply a polyester resin with some talc and pigment for color. So now that we have that straight, you are going to fill the low spots with Evercoats "Everglass," a fine cut fiberglass strand reinforced polyester filler. Get some cardboard and stick it down into the gaps between the hood and the fenders. This is going to be your "levy" for the Everglass. Spread the Everglass on the hood up to this cardboard and press it down into any gaps between the cardboard and edge of the hood. It will blow you away at how much you will fill with a coat of this stuff, if you don’t watch out you will be a quarter inch higher than the finder! After the Everglass hardens, break out the cardboard and sand the Everglass flat to the tops of the fenders. On the edges do the same, “shaping” the panel is in your control. Remember, you are only talking a little bit on the edges, so don't freak out thinking that you have "bondoed" the edge of the hood. It IS the same material, remember. In fact, if you ever sanded a fiberglass part good around the edges you many times will find that the gel coat is VERY thick. I have seen gel coat on the edges an eighth of an inch or more! So just block the Everglass out, feathering it out onto the hood and get it a little low to the fenders. Now, skim coat the whole area with a polyester putty and finish sand it blocking with 120 or 180. When it comes to priming it, polyester primer is like re-gel coating it. It will fill pin holes and really is a great way to finish off fiberglass parts. There is no reason why your fiberglass part shouldn't fit PERFECT. I just did a Pontiac Gran Prix "hot rod" hood with a bunch of scoops and things on it. It was very high cost hood and it fit like CRAP. I did exactly as I have outlined above. It is black, it looks absolutely beautiful and he blew away a his Gran Prix club buddies. Their car’s hoods look like crap now when he parks with them, the darn thing looks like a factory steel GM hood, maybe even fits better!
If you ever saw a Vette with perfect panel gaps, this is how they did it. You don't really think they fit that well from the factory did you?