Body Filler vs Glaze vs Fiberglass Filler: Which One Goes Where

Which one goes where Body filler, glaze and fiberglass filler aren't three versions of the same thing. A repair is built in layers, and each one has a job. Fiberglass filler for strength, where there was a hole. Body filler for the shape. Glaze for the finish. Then primer, then paint. Here's what each one does, how to mix it, and the mistakes that make a repair show through the paint.

Body filler Number one: body filler. Bondo is a 3M brand name, but people use it for almost any body filler. It's for the shape, over metal that's already been straightened. It goes on clean, dry steel, sanded with 40 to 80 grit, and never over rust. Keep it to about a quarter inch. If the dent needs more than that, the metal needs more work first. On galvanized steel or aluminum, check the label. Z-Grip, for one, is rated for both. Not every filler is. Lightweight fillers like these blend in tiny hollow spheres, which makes them lighter and easier to work.

Mixing the hardener Body filler cures with a cream hardener, at about two percent by weight. Evercoat and USC put it this way: a ribbon of hardener, edge to edge, across a 4-inch puddle. 3M says a 3-inch strip for a 3-inch puddle. Each can has its own rule. Too little, and it stays soft. Too much, and you get pinholes, and stains that can bleed through the paint. Fold and press it together on a clean, non-porous board, not cardboard. Stirring whips in air, and that air turns into pinholes. Then you have about three to five minutes to spread it, and it's ready to sand in fifteen to twenty.

Fiberglass filler Number two: fiberglass filler. It's filler reinforced with glass fibers, and it's waterproof. Short strand is for seams, small holes and rust repairs. Long strand bridges holes and rust-through, once the rust is cut back to clean metal. The makers say to finish it with a layer of lightweight filler or glaze, so that's what shapes the top. Short strand on the left, long strand on the right.

Glaze Number three: glaze, also called finishing putty. It's a two-part polyester too, made for the finish coat. It goes over sanded body filler in a thin coat, an eighth of an inch at most, to fill pinholes and sanding scratches. Some glazes are self-leveling, so they flow out smooth. Don't confuse it with one-part spot putty in a tube. That needs no hardener, and goes on in thin coats, each one dry before the next, because thick coats can crack.

Plastic bumpers And if the part is a flexible plastic bumper, skip body filler. The filler makers themselves point you to a flexible product, like these two-part repair kits. Adhesion promoter depends on the kit: 3M's calls for it, and SEM says theirs doesn't need it.

The order, with grits So, the order. Start on clean, dry metal, sanded with 40 to 80 grit, with every bit of rust gone. Fiberglass filler goes on first, only where there's a hole. Then body filler: shape it with about 80 grit, and refine it to about 180. Glaze over that, and finish it somewhere between 180 and 320. Then it's ready for primer. Exact grits vary by product, so check the data sheet.

Five mistakes Five mistakes to avoid. Filler over rust: it goes on clean, bare metal, so grind the rust out first. Piling it on: about a quarter inch is the limit. Mixing on cardboard, which can soak up or contaminate the mix. Stirring instead of folding, which traps air. And guessing the hardener: too much gives pinholes and stains, too little stays

Where to get it soft. Everything in this video is at autofxmart.com: body fillers, fiberglass fillers, and glazes. Each one links its safety data sheet on the product page. Read it, wear gloves, and work with ventilation.

Next: your paint code When it's primed, find your paint, mixed to your car's code, at autofxmart.com. And if you don't know your code yet, this video shows where it is.