Back many years ago, fiberglass was often used to repair rust holes. Today, most folks are welding in patch panels. Why is fiberglass not being used as much today?

Because the rust continues to grow underneath it.

Most of the time when the fiberglass was used, it was applied over the existing rusty metal. because the rusted metal wasn't removed, it kept rotting away and often the fiberglass would simple fall out when the metal it was attached to rusted away.

These days, bonding the new metal to the body shell is the fix. The problem is if the old rusty metal wasn't removed and the new metal was bonded to it, it will fail just like most of the old fiberglass repairs did. If the rusty metal was removed and the new metal was bonded to clean existing metal, the repair should be as good as a welded in panel. Gene

I still use fiberglass & I've been using a "short hair" fiberglass-type "bondo". Poorboy is right, fiberglass mat/resin just slapped on top of rusty metal isn't good. Personally, I have been welding in as many metal patches as possible, but I still need to short hair fiberglass & bondo the seams of the patches to get everything smooth.

No reason fiberglass can't be used. I have repaired several areas that have held up for 20 yrs. As with anything the key is preparation. If metal is prepped and fiberglass is properly applied the metal can't rust. Fiberglass resin applied to clean bare metal would create a bond that will eliminate the possibility of moisture.

Because just the sight of fiberglas tells you there are "issues".

Welders were few and far between back in the day , now everyone has their own mig or tig set up at home ! Gas welding / brazing was the other option.

i have found over the years, brass is hard to get bondo or glass to stick to. it's kind of "greasy", or "oily" is the only definition i can come up with. hard to get it really clean. [at least for me it seems] also, braising induces a bunch of heat that warps panels easily.

Fiberglass was an easy cheap way for high school kids in the 70s 80s even 90s to do a weekend (most of the time awful quality resto)
resto of a rotted out muscle car. Paint it white to not show off the flaws! But when done correctly it lasts a looooong time.

I fixed the base of the rear window in my Charger w fiberglass when I was 18, I'm 55 now and the repair is still working great ! Clean and no rust is the key.

Buying a patch panel and welding it in saves a whole heck of a lot of time. Usually if you're doing a legit repair you already own a small welder so doing repairs with steel is a no brainer. If the metal is really rusted and you have no patch panel, you would generally take metal and shape it fairly close then smooth with filler. With fiberglass you have to lay it over the rusted to blasted remains to give it some shape to hold it up while it cures.

Suddenly, fiberglas became affordable, readily available, easy to use and was widely accepted as a solution. Then we started getting picky as these cars increased in value and about the same time mig welders started to get smaller and more affordable. And when it came to insurance repairs, panel replacement was required.

Thanks for the advice! This is for an old work truck that I really like. Restoration grade work is not needed on this project, but the repair needs to last!

I can access the back of the panel to clean up the rust on the back side. So fiberglass sounds like a workable solution. Thanks again!

The thermal properties of fiberglass are not the same as steel. It heats up and cools much faster thus, it expands and contracts at a different rate. This affects the bond when exposed to rapid heat, cooling and vibration, and there’s the key: Bond. Fiberglass repairs are only as good as the bond. Welding takes two pieces of steel and effectively makes them one. For what it’s worth, if I were replacing rusted metal and couldn’t weld, I would buy a steel repair panel, flange the edge of the area needing repair, and bond it using panel bonding adhesive. If you live in the north, don’t use glass.

before my old man passed, he had a 1990 dodge dynasty front drive. one day before the required state inspection, he came over to my shop asking me to look at his wet carpet under the gas pedal. this was at the end of february, and a ton of snow and salt on the roads. when i looked, there was a big chunk of floor pan missing, and the carpet backing was peeking through the hole. great, just what i needed, a big inspection repair the day before the sticker ran out ! [but that was the old man's way. ]
i jacked up the car, wiped the salty slush off the bottom of the floor pan with a dirty shop rag, and slathered on a bunch of resin around the hole. waited for it to start kicking off, then soaked a patch with resin and slammed it home, tucking some of the patch inside the hole. let that kick, then mixed up some "hair" with extra hardener, then slathered that around until the depression was kinda filled. when that mess started to harden up, and was not moveable, out came the damp weather apply roof tar. smeared that around so it looked like "factory seam sealer" [kinda ]. this took all of about 40 minutes. told pappy to go out & ride around a bit in the slop to get the "patch" dirty before he took it in for a sticker the next day.
that mess set up and lasted three or four years until he traded up to a different ride. looked as "good" then as it did the day i did it, and was rock hard ! sometimes it seems that if you don't care about how you fix something, it lasts forever. when you really try to do a perfect job, it falls apart in a week. go figger !