Do your vehicle's body panels seem to have more Bondo than steel? Is that fender folded in so tight from a rock encounter that a replacement is mandatory? Are the body mounts so rusted out that the body seems to move as much as the suspension? Then maybe it's time for a component replacement, or even the entire body. The question: Replace with steel or fiberglass? Each has its own merits and weaknesses. The quality of the replacement part and your application may be the key factor in making your decision. Poor quality and poor fit can occur regardless of the material used.
Some of the aftermarket steel replacement parts don't really compare well with OEM factory replacements. They may made out of a lighter gauge material, or the alloy content doesn't offer the strength or corrosion properties of an OEM part, or fit and finish aren't all that great.
The same can be said of the poorly made fiberglass part. A poor resin-to-cloth ratio, air bubbles, low-cost resins, worn-out molds or the workmanship itself-all can be contributors to a bad fit or finish, or poor strength. However, if properly made, fiberglass components can be more resistant to trail damage and better looking than their steel counterparts. For instance, look down the side of a steel CJ body. You'll see lots of small indentations, or even high spots, from the spot welds holding the interior pieces in place. The sides of a quality fiberglass body will be straight and dimple-free.
"How well do fiberglass bodies last?'' is a pretty common question. Think for a minute the last time you saw a wood-construction sport boat. Boats take a lot of abuse. So fiberglass can be very tough. Again, let's refer to the quality of manufacturing. As with steel, it comes down to the fact that you get what you pay for. If weight is a criterion, depending on the application, the 'glass part will generally be at least one-third lighter than the corresponding steel part. Super-lightweight fiberglass bodies, fenders, hoods and doors are available for those on a weight-conscious diet.
Disadvantages of fiberglass include the fact that sheetmetal screws may eventually open up their hole, requiring periodical replacement with a larger screw size. Large backing plates are needed to sandwich the fiberglass anywhere you want to mount accessory items. Every electrical connection requires its own separate ground, which most likely isn't such a bad idea, even with a metal body.
The toughest one of all is mounting a rollcage. With no exceptions, the tubing must be attached directly to the frame. The common practice is to use standard manufactured bars and then support them with either a kit supplied by the body manufacturer or some type of home-made braces. If you doubt your ability to do this, get professional help. Outriggers from the frame, similar to body mounts, can be fabricated for support. The best bet would be to build a custom cage that tied into the frame in at least six places. Not only would you gain rollover protection, but the cage structure would strengthen the frame.
Story time: Back about 1981, I bought a 4WD Hardware 'glass body tub from a now defunct West Coast 4x4 shop. Over the years, the body held up remarkably well. The panels are smooth, and the gelcoat finish is easy to maintain since it's not painted, so I don't worry about brush scratches. It's rust-free, lightweight, dent-resistant and it doesn't flex.