Best Glues for HDU High Density Urethane

By Jim Hingst 

If you fabricate and install an HDU High Density Urethane sign according to manufacturer’s instructions, it should last between 10 and 30 years. One of the keys to creating a durable HDU sign is using adhesives, texturing materials, primers and paints which are compatible with the signboard. That’s why I selected the Precision Board materials by Coastal Enterprises. They have a complete system of products matched for compatibility. What’s more, if you have questions, they have the service organization that provides prompt, reliable answers.

Two best glues to use when gluing High Density Urethane are polyurethane (urethane) glue and epoxy resin glue. The most popular urethane glue on the market is the original Gorilla Glue. Today the Gorilla Glue company manufactures many different types of glues.

I have used Gorilla Glue and the urethane glue that Coastal Enterprises sells when gluing multiple sheets of High Density Urethane together to make one thick block for carving.  In the picture below, I have glued four sheets of HDU together and then rough cut the shape with a sawzall Reciprocating Saw. See the photo below.

Reciprocating saw or Sawzall can help to rough out a carving.

● A coarse reciprocating blade easily rough cuts a thick block of HDU.

● When rough cutting a block of HDU clamp the block to the work surface.

Milwaukee Super Sawzall Reciprocating Saw

● The Original Sawzall on the market.

●  Rated the strongest most powerful sawzall.

● Works with a variety of blades covering a range of construction and demolition applications.

● Great for trimming branches.

● Rugged construction for  years of reliable performance.

 I then shaped the block using gouges to create my Japanese Oni mask shown below. To the top of this head I will glue horns. For that application, I will use an epoxy adhesive. 

The Original Gorilla Glue has a number of great features:

● It has tremendous bonding power.

● It is 100% waterproof, so it is a great for building outdoor HDU signs.

● Using water activates the glue so it expands to fill any minor gaps between boards.

Bonding HDU Boards Together Using  Urethane Glue

What’s nice about polyurethane adhesive is that there is no measuring. To activate the urethane glue you just need to moisten the surface to be bonded by lightly spraying the HDU with water, then clamp the pieces and you are done.  More water is not better, so wipe off any excess water with your hand.

In creating a Tiki head for a sign, I glued sheets of HDU together. Prior to gluing the pieces together, make sure that the surfaces fit tightly so you do not have any gaps.

After the surfaces are moistened, apply a thin coating of urethane glue using an inexpensive squeegee. As you spread the adhesive, squeegee the excess to the center of the board. By doing this, you this technique will prevent air pockets from being trapped between the boards.

After the urethane glue is applied, position one board over another. Apply weights to the top of the boards so that the pressure is evenly distributed. See the photo below. Using these weights alone is not sufficient to hold the boards in place. To keep them from slipping and sliding out of place, toe nail the boards together in several places with drywall screws.

Urethane glue can expand near four times their original volume. In a couple of minutes the adhesive will start to bubble.  Keep the surfaces clamped for at least four hours. Full curing occurs after 12 hours. You can expect some squeeze out. Remove the residual adhesive with either a razor or a #3 woodcarving gouge.

If you have not worked with urethane glues, be advised that they are messy. For this reason wear latex gloves and your grubbiest clothes. Also cover your work table with application tape for easy clean up. Placing sheets of wax paper or clear application tape underneath the glued pieces prevents gluing your HDU project to your work surface.

Gluing with Epoxy Adhesive

While signs made from HDU have a number of advantages over redwood and cedar, one disadvantage, is that it has neither the density nor structural strength of wood. As a comparison, the density redwood is about 27 lb. per cubic foot, which is nearly twice that of 15 lb. HDU.

Whereas wood is comprised of long, sinewy fibers, HDU is made up of very tiny cells – bubbles, if you will, that are formed in the chemical foaming process. These little bubbles don’t provide a lot of structural strength.

If a sign is subjected to significant wind load, you must support the HDU board in some way to strengthen the panel. For the background of my Tiki Bar sign I used a one and a half inch sheet of HDU, which I carved into as much as an inch deep in some places. That didn’t leave me with much bulk the sheet.  I felt that I needed extra strength so I laminated a 2’ x 4’ sheet of aluminum composite panel (otherwise known as Dibond) to a sheet of HDU of the same size. For this application the Dibond was a better choice than wood or MDO, because it is not only lighter weight but more importantly it is more dimensionally stable.

A urethane adhesive would have worked, but epoxy adhesive provides better structural strength and is great for bonding HDU to wood, plastic or metal.

The West System Epoxy is comprised of three components: the 105 resin, a hardener and, often when used as an adhesive, a filler. The key ingredient in any of these formulations is the West System 105 resin. Depending on which hardener and filler you select, you can use it to as an epoxy glue, or as a clear coat or for laminating fiberglass cloth or to repair damage to a surface.

West System 105 Epoxy Resin

For my project I selected  the West System 205 Fast Hardener. If you need additional working time, especially when working in warmer environments, use the West System 206 Slow Epoxy Hardener. For coating a bar top or wood project, you can use the West System 207 Special Coating Hardener.

What’s important when working with any epoxy system is that the resin and the hardener are mixed in the correct proportions. If you happen to mess up the mixture ratios, the resin will not cure and you will end up with a mess on your hands.

Metering out the right proportions is not difficult if you use the West System 300 mini pumps. All you need to remember is to combine one full pump of resin with one full pump of hardener.

West System 205 Hardener

After dispensing the two components in a clean container, you must thoroughly mix the West System Epoxy resin and hardener together. Whenever you mix any components, make sure that you scrape the sides and the corner at the bottom of the container.

To change the consistency of the resin/hardener mixture, you can add filler. It can also be used as an extender. Selecting the right filler for your application is the confusing part of using epoxies because there are so many products from which to choose. I used the West System Microfiber Filler, which consists of microfibers of chopped cotton. Using this filler creates a good multi-purpose adhesive.

West System MICROFIBERS - 20 OZ

If the surfaces are uneven, a filler also produces a mixture that will fill any gaps. The high density fillers, such as West System’s 405 or 406, change the structural characteristics of the epoxy, making it either stronger or thicker. These high density fillers become very hard after the epoxy cures, which makes sanding difficult. The different fillers can be mixed together to produce your own special concoction.

In thickening epoxy, add small amounts of the filler, little by little, until you produce the desired consistency. For laminating flat panels of HDU and aluminum composite panel, you will want the epoxy to have the consistency of ketchup.

Safety Recommendations. While epoxy poses little potential for respiratory problems, you should work in a clean, well-ventilated shop.  It also would not hurt you to wear a respirator with an organic vapor canister. Contact with your skin can result in dermal reactions such as skin irritation, rashes and allergic dermatitis. For this reason, always wear the chemical-resistant gloves, clothing and other safety gear recommended by the manufacturer. If you are sanding, wear a respirator with a particulate filter.

Guidelines for Working with Epoxy

When bonding one surface to another with epoxy adhesive you need to follow a few guidelines:

● Because there are  time limitations when working with epoxy glue, make sure that you assemble all of the materials and equipment that you need for the job. Take into consideration, that when you would with larger batches or  thicken an epoxy with filler, you increase the mass of the mixture generating more heat, which shortens the pot life.

● Test, Don’t Guess. As a word of caution, if you are bonding two different types of materials it is best to test the bond strength of the dissimilar materials prior to fabrication. While epoxy glue sticks to just about anything, there are some low energy plastics, such as polyethylene, that it will not stick to.

● Make sure the surfaces are dry.

● In storage and during fabrication, the surface of the HDU can accumulate dirt and dust. To clean the surface, slightly abrade the surface with a grey Scotch-Brite pad.  Before applying the epoxy adhesive, brush off any residue from the surface using a chip brush and wipe it down with a clean rag. Contaminants will compromise the adhesive bond.

● When bonding to a non-porous smooth surface, such as the aluminum composite panel, rough it up with 80-grit sand paper. This will give the surface some tooth so the adhesive bonds to it.

● To bond the HDU and the Dibond panel, used a two-step procedure. First, I coated the HDU with epoxy without any filler using a foam roller. This ensures that the epoxy adhesive wets out the entire surface of the HDU.

● Right after applying this first coating, pour the epoxy glue thickened with the filler onto the HDU. If you are covering a fairly large surface pour the thickened epoxy in the middle of the panel and spread it evenly using a notched spreader. You do not need much adhesive to create a great bond. This type of spreader applies a uniform thickness of epoxy adhesive and prevents air pockets from developing in your lamination.

● After coating epoxy adhesive to one panel, laminate the two panels together using moderate pressure – just enough for the thickened adhesive to start squeeze out. An alternative to using clamps to secure the panels as the epoxy cures is to use drywall screws and weights evenly distributed over the panel. Too much clamping pressure will squeeze out too much of the adhesive to from a good bond. It is critical that you maintain pressure the two parts to be joined until the epoxies cures, which is generally about eight hours. Failure to maintain consistent pressure may result in the parts pulling apart, which will stress the bond.

● Clean up any of the adhesive squeeze out, as best you can, before it hardens.

Pot Life and Working Time

Pot life and working times for epoxy glue can vary greatly based on the temperature of your shop. The warmer the temperature is, the faster the epoxy cures. At 72°F (22°C) pot life for a mixture can as little as 10 minutes. At 90°F (32°C) that time is cut in half. That means that if you are working on a hot day, you better work fast!

It case you don’t know what pot life is, it is the amount of time that it takes for resin and hardener to cure in your mixing cup before it hardens. Working time, on the other hand, is the time it takes for resin to cure after it is spread out onto the surfaces that you are trying to bond. Depending on the ambient temperature, you can have as much as an hour for working.

Curing to a hardened state can take as much as 6 to 8 hours. Complete curing takes even longer. In about two weeks, epoxy adhesive is nearly at full strength. (Don’t be concerned if the adhesive on a test panel feels tacky before it cures completely. Epoxy adhesive should harden as solid as a rock, if you follow the manufacturer’s directions.)

So why is there such a difference between pot life and working time? Mixing resin and hardener creates a chemical reaction that creates heat. In a confined space such as a mixing cup, you contain the heat. When you do that, the heat builds up really fast. In fact, you can feel the heat as you are holding the mixing cup. In most cases it is better to mix multiple smaller batches than one big batch.

When you pour the mixed epoxy glue onto your working surface, the heat dissipates. Using the example of bonding Dibond aluminum composite material to the Precision Board HDU, the metal surface can dissipate the heat fast. On the other hand, the HDU does not dissipate the heat so fast.

So what can you do? If it is impractical or impossible to control the temperature of your work environment, the simplest solution is to use a slow hardener. If you are mixing on a large surface, you can mix smaller batches until the surface is completely covered.

About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.

Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer.  Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.

Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for  publications, such as  Signs Canada, SignCraft,  Signs of the Times, Screen Printing, Sign and Digital Graphics and  Sign Builder Illustrated. He also posted more than 400 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published.  Vinyl Sign Techniques is available at sign supply distributors and at Amazon. 

Slides of Jim's Artwork

© 2018 Jim Hingst