The Best Epoxy For Secure Coating

The guide below contains the most commonly used types of epoxy, and a brief description of the intended use of each. This is by no means an exhaustive list and there are many specialty epoxies out there, so make sure to read each one and find the product that is right for you and the task at hand.

All-Purpose Epoxy

There are special purpose epoxy adhesives for all sorts of applications. But for most projects and repairs, a general purpose epoxy works just fine. Unless there's some reason to use a special purpose epoxy, you might as well use the all-purpose ones and save yourself the extra cost of those specialty adhesives.

In reality, the difference in an all-purpose epoxy and any specialty epoxy is minimal. That's not to say that there's no place for specialty epoxies, but in most applications the extra mile those adhesives go really isn't really necessary. Epoxy of any sort is one of the best, most powerful adhesives available so it's usually overkill for most projects. With that being the case, using a specialty epoxy can actually be nothing more than adding to the overkill.

The big advantage of sticking to an all-purpose epoxy is you only need to keep one product in your workshop, rather than a collection of them. That one product will do most of what you need, making it an ideal overall choice.

By minimizing the number of different adhesives that you stock, you justify going for larger containers of your all-purpose epoxy. That will save you a considerable amount of money, as well as reducing waste. I don't know about you, but I rarely make it through tubes of epoxy or the twin plunger pack, without the tubes getting plugged up, the cap getting stuck or the tubes breaking. When that happens, I end up throwing the rest away.

On the other hand, using larger containers (such as four or eight ounce bottles) is actually easier. Not only do you not run into shortages, but I have yet to have a problem with the package. So, even though it sits in my workshop longer, I get to use it all.

Like all epoxies, these are intended to be used in a 1:1 ratio, mixing equal parts of the two components. That makes it easy to get the right amount of material, each and every time. Always mix the two components thoroughly before application, so that they can begin to react and cure. If any part is unmixed, it will not cure and have to be cleaned up.

Wood Epoxy

Bonding wood can be a challenge, as compared to other substrates where epoxy might be used. The major issue with wood is that the substrate is often weaker than the adhesive being used to fasten it. This varies greatly, depending on the wood selected, as not all types of wood have the same structural strength.

Countering that is the fact that wood is a very easy substrate for adhesives to bond to. Although usually smaller than the human eye can see, wood is porous, allowing the epoxy to enter into the pores and get a good "grip" on it. This is magnified with open grain woods, like oak, which have more surface area for the epoxy to grip to. So, chances are that the epoxy isn't going to come loose of the wood. In fact, if the joint fails, it will be the wood itself that fails, not the adhesive to wood interface.

For this to work, it's usually best to use a low viscosity epoxy formulation, which will allow the epoxy to soak into the pores well. Coat surfaces well, before attaching, to ensure that the entire available surface area is used. Avoid using epoxy that has started to cure, as it won't soak into the wood pores and fiber.

While epoxy isn't used all that often with wood, there are some applications where it is an excellent combination. More than anything, we find it used in marine applications. Boats often have wood structures under their fiberglass skins. The two are often bonded together with epoxy, as using fasteners would cause the fastener heads to pull through the fiberglass.

In addition to using epoxy as an adhesive for wood, it can also be used as wood filler to combat damage, either due to accident, vibration or rot. In all cases, it is necessary to remove the damaged wood, so that the epoxy can make contact with structurally sound wood. While epoxy can help to stabilize damaged wood, it will not have any true structural strength. For that, it has to adhere to wood that has not been damaged.

When using epoxy to attach wood parts which are going to be finished, especially with stains and clear finishes like varnish, be sure to apply a coat of the finish to the wood, before using the adhesive. The epoxy's ability to soak into the pores of the wood will make it so that the finish cannot. This will cause places in the work which won't accept the wood stain, but look like they have been stained by some other type of finish.

Metal Epoxy

Perhaps one of the hardest materials to glue together is metals as there are two basic issues here. The first is having a surface that the adhesive can stick to, and the second is having an adhesive that's strong enough to provide a good metal-to-metal bond.

Bonding metals together with adhesives requires a different way of looking at the whole idea of joining metals. Typically, we join metals together with either fasteners or welding. In either case, only a small surface area is used for the bond. It's assumed that the strength of the metal itself will allow that small surface area to be sufficient, as the metal itself will spread the stress over a large area.

When using epoxy or other adhesives with metals, the joining area must be larger. You can't simply do an edge connection and expect it to hold, nor can you do a single spot by replacing a bolt or spot weld and then expect it to hold securely. The entire available surface area must be used in order to allow the greatest possible tensile strength for the adhesive. A large area of adhesives will obviously be considerably stronger than a small one, and will spread any forces to disassemble the joint across the entire area.

In this manner, two pieces of metal joined together by epoxy can be almost as strong as one piece. In restoration of vehicles, it's quite common for metal parts to be rusted thin to the point of not having enough structural strength. Bonding another layer of metal to the original part with the use of a good epoxy can provide as much strength as replacing the part itself. This type of repair is especially useful when the original part can't be removed for replacement.

The other issue which comes up, a surface that the glue can adhere to, requires a combination of two things. First of all, the surface needs to be clean of oils. Few adhesives will stick to oil and metal parts are often oily from their manufacturing and processing. The other thing that helps the bonding process is to rough the surface with a sander or grinder, giving the adhesive some "bite" into the metal. This rougher surface prevents the adhesive from peeling off by forcing it to conform to the surface.

Epoxies uses for metal must have some gap filling capability, as the metal parts often don't fit together perfectly; the adhesive must bond to both surfaces as well as fill the space between them. Epoxies are excellent for this as they are heavy-bodied. Some of these have metal filler in the epoxy as well which further improves their gap filling capability.

Epoxies for metals can be both two-part liquid and epoxy putties. Where there's a need to replace missing material, epoxy putty is a better choice. In cases where the metal parts fit tightly together, use of a two-part liquid is better.

Once cured, these epoxies are often drillable and even threadable for fasteners. This must be done with care, as the epoxy is not as strong as the original metal. However, threaded fasteners can be used, especially when the fastener is also holding to the metal parts.

Plastic Epoxy

Plastics can be difficult to bond together due to their variety in hardness, texture and natural oiliness. But getting the plastics to bond isn't the whole battle. Once bonded, many plastics will fail either at the bond itself or at the edge of the adhesive. A flexible adhesive is needed to prevent adding stress to the parts which will ensure they'll likely break again.

While there are some adhesives designed specifically for plastics, they're usually designed with one specific type of plastic in mind. In most cases, these adhesives work well with that particular plastic and under specific parameters. However, they're rarely able to handle high stresses.

On the other hand, epoxy is a very strong adhesive with a high tensile strength which makes it excellent for pretty much all types of plastics. As a two-part adhesive it's not affected by the plastic itself, being totally self-contained as far as curing is concerned. Cured epoxy will often actually make a plastic part stronger than it was before.

You can even use epoxy for edge gluing of plastics, one of the hardest bonds to accomplish, due to the small surface area. Typically, edge glued plastics separate easily as soon as any lateral pressure or bending is applied. This is much less likely to happen with use of an epoxy product.

For additional strength, epoxy can be used in conjunction with fiberglass cloth. This is exceptionally good when high strength is needed for parts that are edge glued together (such as a fitting being glued to a tank) without an adequate flange. Sections of plastic pipe can be connected together in this way or repairs can be made to recreational vehicle waste storage tanks. The epoxy is more flexible than fiberglass so it isn't brittle, even with the fiberglass cloth reinforcement.

Some plastics, such as polyethylene, polypropylene, and Teflon, are designed to provide non-stick surfaces. No adhesives should be used with these types of plastics and that includes epoxy.

Typically, a fast cure time is desirable when working with plastics as it's hard to hold the parts together or clamp them firmly. It's important to clean the surfaces before bonding them, especially when working with oily plastics like nylon. However, any molded plastics can be oily due to the mold release compounds used in their manufacture.

Putty Epoxy

Epoxies are great for many situations, but they are not gap filling. While additives can be put in the epoxy to make it gap filling, few gap filling epoxies are available on the market. For small jobs, most people turn to epoxy putty.

Epoxy putty is a two-part plastic adhesive, made of the same basic chemicals as epoxy. The main difference is that the putty is solid, rather than liquid like other epoxies. This makes it possible to be used for gap filling needs, as well as places where structural strength needs to be added. When epoxy putty is wrapped around a joint and allowed to harden, it strengthens that joint, much like putting a cast on a broken bone.

Once cured, epoxy putties are at least somewhat machinable, meaning that it can be drilled, sanded, ground and even tapped to put screws into. Some manufacturers claim that their epoxy putties are as strong as steel. This is true, but only as measured by shore rating (hardness), not tensile or sheer strength.

The biggest problem with using epoxy putty is that of adhesion. All adhesives gain most of their adhesion from filling in the grain in the substrate material and then bonding to it. The irregularities in the surface provide shear strength to the bond. Since epoxy putty is in a putty form, it doesn't flow into the materials natural grain, adhering to it. Rather, it is necessary to push the epoxy putty into the grain, so that it can grab.

The other problem is that of ensuring a clean surface for the putty to adhere to. Liquid epoxies may displace grease or dirt on the substrate surface, or even mix them, but epoxy putty won't. It needs a clean surface, free of grease dirt or other solvents. Failure to provide a clean surface is the most common cause of failure when working with epoxy putty.

Epoxy putty is commonly sold in sticks of about two ounces, but is also available in larger quantities for industrial or commercial purposes. If you use a fair amount of it, it is ultimately cheaper to buy it in these larger quantities.

Waterproof Epoxy

While epoxies are some of the strongest adhesives in the world, they aren't perfect and have their weaknesses. One of those weaknesses is dealing with water. While water won't fully dissolve epoxy, it does affect an epoxies structure and ultimately weakens it. This makes it so that epoxies are not waterproof, but merely water resistant. Therefore, there's a need to add a stabilizer to the epoxy when used in applications where the epoxy is likely to be submerged in water.

These additives are normally added to the epoxy at the factory, rather than in the field. However, it's possible to buy them and add them yourself if you like the extra work the process entails. Usually the easiest way to make sure epoxy has waterproof capabilities is to buy ones that already have the additive in them.

Generally speaking, these waterproof epoxies are referred to as "marine epoxies" because they are primarily used for the manufacture and maintenance of boats. All marine epoxies are water stabilized, ensuring that their bond isn't compromised by continual submersion in water.

One nice thing about marine epoxies is they come in a wide variety of forms. Rather than just liquid and putty, we can also find gel and paste epoxies which provide greater gap filing capabilities than liquid epoxies while being easier to form than putty epoxies. There are also low viscous epoxies which can be used as a protective coating for teak or other woods used in marine applications.

Like all other epoxies, these are two-part adhesives which must be mixed to cure. Curing is a chemical reaction rather than the drying most adhesives depend upon. This means that these adhesives will cure in any environment, even when they're submerged in water.

Marine epoxies are often high gloss so they can be used without having to finish them. However, like other epoxies they can be sanded, drilled and cut, once they've set. This allows forming the final part (including the epoxy) as if it were one solid piece. When painting glossy-finished epoxies it's a good idea to sand the epoxy before applying the paint so as to ensure good adhesion by the paint.