Fiberglass Resin, Mat and Cloth: How to Fix a Rust Hole (By the Sheet)

A rust hole takes fiberglass A rust hole doesn't take body filler. It takes fiberglass: polyester resin, and a mat or a cloth laid over the hole. Mat where you need strength, cloth where the skin has to come out smooth, and a resin that starts to set minutes after the hardener goes in. Here's the lay-up by the makers' own data sheets: the margin, the drops, the clock, and the mistakes. Everything in it is at autofxmart.com, link below.

Mat or cloth? First, which one. Mat is non-woven: random strands pressed together, so they interlock and span a gap. Cloth is woven, threads over and under, like fabric. The mat is thicker. 3M says that's the one when you need more strength, bigger holes and cracks; the cloth is thinner and more flexible, and it gives a smoother finish. USC's resin sheet says mat or cloth over rust-outs, holes, tears and cracks, and its filler sheet puts cloth or screen under the filler on a large hole. And a small hole, a crack or a dent? The same sheet says no backing needed: fiberglass filler on its own does it. So: mat over the hole for strength, cloth where the surface has to come out smooth. Either one takes the same resin.

Under the lens Under the lens: the mat is chopped strands held by a binder, the cloth is threads over and under. Both wet out with the same resin, and the resin is what sets.

What you're building Here's what you're building, cut through. Steel with a hole, and two inches of bare metal around it. A layer of mat, wetted through with resin, two inches past the hole. A second layer, a little larger, while the first is still tacky. Then the patch is sanded and skimmed with filler, and primer goes over the lot. Here's each step, from the sheets.

Grind it back: the 2-inch margin First the margin. Every sheet says the same thing: grind everything off, down to bare metal, two inches past the damage. USC says two; 3M says two to three; Evercoat says several, beveled toward the centre. Eighty grit, or a forty-grit disc on a grinder. Then clean it and dry it. And if you can get behind the panel, do both sides: USC says it adds strength. Evercoat backs the hole with cardboard under wax paper, taped on behind, so the first layer has something to lie on.

The resin sheet Here's USC's resin sheet. Fourteen drops of liquid hardener per ounce of resin. A two-ounce tube does the whole gallon; a quarter can takes a quarter tube. Resin and hardener at room temperature, sixty-five degrees at least. Nine to fifteen minutes of gel time at seventy-seven. Sand at about an hour. And keep the sanded fiberglass to an eighth of an inch. Bondo's sheet counts ten drops an ounce, with eight to twelve minutes to work it. Evercoat's counts twelve at room temperature, eighteen when the shop is cold, ten when it's hot. The number is the can's: read yours.

Mixing: count the drops Pour what you'll use into a clean cup. Not the can: 3M's sheet says don't mix hardener in the can, and every sheet says never pour the mix back, because it hardens the rest. Then count the drops. USC says fourteen per ounce. Bondo says ten. Evercoat says twelve at room temperature, eighteen when it's cold, ten when it's hot. Stir it gently, thirty to forty-five seconds, scraping the sides of the cup. Evercoat says one to three minutes, until the colour shifts. Too much hardener or too little, USC says, and you get poor adhesion and a soft cure.

The clock starts at the first drop From the first drop, the clock runs. USC gives its resin nine to fifteen minutes of gel time at seventy-seven degrees. Bondo: eight to twelve minutes of working time at seventy-five. Evercoat: four to seven minutes to work it, and twenty to thirty before it gels. USC's gel, the one for vertical panels, eight to fifteen. So mix only what you can use in that window. It's hard enough to sand at about an hour on USC's sheet, one and a half to two on Evercoat's, two on Bondo's, and cooler air makes all of them longer.

Cut it bigger than the hole Cut the mat or the cloth bigger than the hole: Evercoat says at least two inches past it all round, 3M two to three. If the hole takes more than one layer, cut each piece a little larger than the one under it, and with cloth, Evercoat turns each layer forty-five degrees for strength.

Wet it out and lay it over the hole USC's method. Lay the piece on foil, or a poly bag, and pour the mix over it with a brush or a spreader until it's completely saturated. Evercoat says work it in until the fibres disappear, and use only enough resin to do that. Then lift it onto the hole, and go over it with a spreader: flatten it, and work out every air bubble. When the resin has set, the foil peels off. 3M does it the other way round: brush a coat of mixed resin onto the panel first, lay the piece into it to tack it, then brush more resin through it with a dabbing motion. The dabbing is what pushes the air pockets out. The next layer goes on while the first is still wet or tacky. If it's gone hard, 3M says sand it with eighty grit before the next one.

Vertical panels: the gel, and the dip On a vertical panel, USC's resin gel does the same job without running. It mixes with cream hardener, two percent by weight, and about six ounces of mixed gel does a square foot of mat. Coat one side of the piece, place it wet side down, squeegee the air out from the middle, and coat the top. Then press the centre into a slight dip. That leaves room for the next coat. Give it ten to fifteen minutes, and build the rest up to the panel's contour with

Cured: sand it, skim it, prime it filler. Once it's hard, it sands, grinds and files like steel. USC says eighty to one-eighty grit; Evercoat eighty, then one-eighty; 3M one-eighty, and featheredge the edges into the panel. Skim the low spots and the pinholes with a little mixed resin or a body filler, keep the fiberglass to an eighth of an inch sanded, and then it's ready for primer. One more from 3M: a heat lamp eighteen inches away for fifteen minutes speeds the cure, but too much heat cracks it. And Evercoat says heavy brushing or direct sun can leave the surface tacky.

Which can: resin, gel, All Resin, or filler Which can, then. Plain fiberglass resin wets out mat and cloth fast: a hole in a hood, a floor, a flat panel. The gel is for vertical panels; USC's sheet says it won't run or sag. All Resin is the one for SMC and fiberglass panels, mixed with cream hardener at four percent by weight. And for a small hole, a crack or a dent, the Duraglas sheet says skip the backing: fiberglass filler on its own.

Five mistakes Five mistakes. Mixing in the can: the leftover hardens, so use a clean cup, and never pour the mix back. Guessing the drops: too much or too little gives poor adhesion and a soft cure. Skipping the margin: resin over paint or rust lets go. Bare metal, two inches past the damage. Leaving air under it: every sheet says squeegee or dab the bubbles out before it gels. And mixing too much: the clock is eight to fifteen minutes, so mix what you'll use.

Where to get it Everything in this video is at autofxmart.com: the resins and their hardeners, mat and cloth, and the fiberglass fillers. The sheets are written for trained professionals, and every product page links its safety data sheet: gloves, eye protection, ventilation, and a dust mask when you sand. Read it first.

Next: the primer, then your colour When it's primed, find your paint, mixed to your car's code, at autofxmart.com. And which primer goes over a repair like this one is its own video.