Finding The Right Fill

April 3, 2003

Duct tape, besides protecting you from a terrorist attack, fixes just about anything — but don’t try it on stone.
Sounds silly? If you pick the wrong product to fill a crack, fissure or small chip, you’ll end up with the same results as a roll of the Big Grey … even if it’s designed to work on stone.
Plenty of manufacturers make material that’s easy to use, regardless of whether you’re working indoors or outdoors with granite, marble or limestone. Pick the wrong product, or fail to use it properly, and the end result can be a fill that doesn’t fully penetrate the surface that needs to be repaired, or — over time — will discolor, be brittle, or even fail to cure entirely.

MULTIPLE CHOICE
Chips and cracks in natural stone had ancient builders cursing long before they had writing materials and an alphabet to record their woes. Today’s most popular solutions are fairly recent developments, owing their existence to 20th-century chemical research.
The first chemical product to come on the scene, and the one still extensively used in the industry, is polyester.
“The polyesters are an organic, manmade material,” says Mike Beckmann, president of the Mukwanago, Wis.-based Bonstone Materials Corp. “They work well with interior stone, but if you use them outdoors they don’t do well because they’re not resistant to weather extremes and ultraviolet (UV) light. They tend to shrink, and when you have that on top of environmental conditions, they tend to pop out.”
Acrylic products followed a few years later. At low viscosity, the acrylics are a popular way to consolidate cracked stone because of penetrating ability, says John Maher, business development manager for the Eaton Rapids, Mich.-based Axson-North America Inc.
Other advantages of the acrylics are that they’re more color-stable and more resistant to sunlight, but performance is similar to the polyesters in a couple important areas.
“The speed of the material is good,” Maher says. “You don’t have to wait for 24 hours like you would with an epoxy. There’s also its ability to take a polish. The polyesters and the acrylics both polish out to a very high gloss, which matches the stone quite well once it’s finished.”
Epoxies are a still newer kid on the block. And, as Maher notes, their biggest drawback is often viewed as the slowness with which they set and cure, and – with two-part epoxies – achieving the proper ratio of resin to hardener. While different formulations perform differently, the times involved with epoxy are often listed as hours, rather than minutes, which can be a real drawback if you need to finish a job.
The epoxies have proven popular at the quarry level because they offer a good reinforcement for fragile stones. For installers, it’s important to realize they’re also much less prone to shrinkage than the polyesters, but — depending on the formulation — they may discolor when exposed to UV rays in sunlight.
Bonstone, for one, thinks a newer product may provide a better answer than any of the other three for exterior and interior patching. The company is awaiting a patent for its polyurethane products, which utilize chemistry similar to what is used to automotive paint.
“It has the benefits of an epoxy, plus it’s color-stable,” says Paul Klees, Bonstone sales manager. “We believe it’s the ideal product for patching.”
Besides considering the environment in which the filled stone is going to be located, it’s also important to look at the viscosity of the fill material. While a knife-grade or medium viscosity is helpful for patching or adhering two pieces of stone (and, in some applications, bonding scrim to the back of stone), small crack-fill jobs generally require a much-thinner viscosity.
Fred Ignatovich, president of the Byron Center, Mich.-based Superior Stone Products, says his company recommends its water-thin acrylic for such jobs, because of its ability to penetrate deeply into the stone.
“You need the adhesive to go where it needs to go,” he says. “You want something very thin and with a flowing consistency.”
Another important consideration in such situations, Ignatovich adds, is the cure time. A longer cure time will allow the product to fully penetrate and consolidate the stone.
Robert Theofield, sales manager with the Holbrook, N.Y.-based Akemi North America, agrees. He says a product with a fine viscosity can help with micro fissures; on larger cracks, something with the same viscosity wouldn’t be effective because, “it would be like turning on the faucet and watching the water go down the drain.”
“Every job is a little bit different,” he adds.

A LITTLE KNOWLEDGE
While selecting the right product for each particular job should ensure a certain level of success – and it does – there are still problems that can crop up with a job if you don’t pay attention to the details.
It sounds pretty elementary, but Andrew Landis, sales manager for Plastic Associates Manufacturing Inc. in Norcross, Ga., says one thing people often overlook is making sure components are thoroughly mixed, whether with polyesters or epoxies.
“Because these are chemical reactions, you need to have all the particles disbursed within the solutions, and then you have to make sure you thoroughly mix both solutions,” he says. “With the polyesters, the trick is to use enough hardener, depending on the temperature.”
Akemi’s Theofield agrees. He says the same holds true when you’re working with the A and B components of a two-part epoxy. He relates the story of a customer who had used an Akemi epoxy to restore an outdoor fountain; when he was called in a second time after the fountain had been damaged, he wondered why the epoxy had yellowed.
“We sent it to our lab to be analyzed, and did cross-sections,” he says. “We found the customer hadn’t mixed the ratios correctly, and — after more than a year — it still hadn’t cured. It was still soft in the center, even though it was hard on the outside.”
Getting things mixed thoroughly is one thing, but hitting on the exact ratios for best results with these products can be a challenge. It’s less of a problem with polyesters than with epoxies; with polyester, the ratio of hardener to resin is two percent to four percent.
“If you put in four percent, it will cure faster than if you put in two percent,” says Theofield. “In New York in the winter we may want to use four percent because it’s colder and we went it to cure faster. In Florida or Texas in the summer, when it’s hot and humid, we’d want to use two percent.”
Hitting the correct ratios with epoxies is even more critical, as Theofield’s customer found out. Different ratios can speed up cure time and lower cure temperature with epoxies, but the end result may be something that’s brittle, or not fully cured.
“With an epoxy, you have to mix it in an exact ratio, says Superior’s Ignatovich. “I have a scale in the lab that I use, but in the field it has to be done by eye and it’s difficult. However, if you go more than 10 percent off in a mix, you’re going to run into problems.”
One option to avoid that is read the technical specification sheets that come with the products. Another possibility is working with single-component epoxies.
Even if you’re not interested in adjusting your cure time through changing the ratios, it’s also important to remember that temperature plays a big role in how well these products work, since all of them are temperature-sensitive.
“The colder the temperature, the slower the cure; the warmer the temperature, the faster the cure,” says Bonstone’s Klees. “A good rule of thumb is that your cure speed will approximately double with every 15º of (Fahrenheit) temperature. A job at 75º will cure twice as fast as one done at 60º, and it will cure twice as fast again at 90º.” (Most epoxies won’t cure under 50º unless heat is applied.)
Different manufacturers set different levels below which their products just don’t work. However, it’s always possible to warm up the stone to be filled before doing a job, using anything from a hair dryer to a heat gun to a blowtorch.
Still another important ingredient for a successful fill: having a clean surface with which to work. Klees says that in all cases, it’s important not to be working on any surface that may be oil-stained , dusty or laden with frost or ice. Akemi’s Theofield suggests cleaning it out with water or blowing it with air. The latter is particularly helpful, since the surface to be filled needs to be dry.
Ignatovich suggests acetone.
“Acetone is a recommended solvent for cleaning stone; it tends to pull a lot of moisture with it, and it’s excellent for its compatibility with a lot of adhesives,” he says.
Plastic Associates’ Landis says not only does he recommend cleaning the surface thoroughly with a commercial stone cleaner, but then applying a seal to the surrounding stone before doing the fill.
“If you seal the stone against water and other problems before you do the actual application, the excess will clean off a little better,” Landis says.
Ultimately, crack filling isn’t much more than a marriage of a bit of artistic talent and that high school chemistry class, says Theofield. Each company’s product tech sheets go a long way toward explaining uses, cure times and temperatures. And, if anyone ever has a question about a particular job, the best advice is calling the manufacturer, he says.
“We field a lot of phone calls on technical questions all the time,” he says. “If you’re not sure about something, don’t wreck several hundred dollars, or even several thousand dollars worth of stone. Go ahead and call us. We’re more than happy to help out.”