Many outdoor projects require the use of an adhesive or glue. Outdoor weather conditions in North America are extremely varied from north to south and east to west. Your outdoor project can be exposed to torrential rains, high humidity levels, excessive temperatures, very low temperatures, dramatic temperature swings and very dry periods.
Figure 1 - Polyurethane glue
Polyurethane glue, as shown in Figure 1, is one of the few adhesives that can withstand the extreme North American weather conditions. Its application is somewhat different to most adhesives.
Most home handymen are familiar with the common use of polyurethane, that being as a finish that is applied to wood both outdoors and indoors. Polyurethane glue has no relationship to the polyurethane finishes. Unlike many of the outdoor adhesives which are two parts and require that they be mixed together; polyurethane glue is a homogeneous product that is not mixed with other products.
Using polyurethane glue
If you have not worked with polyurethane glue you may be somewhat surprised to learn that its method of curing is quite different from its outdoor adhesive competitors. Most adhesives cure by evaporation or chemical reaction. Polyurethane glue, on the other hand, reacts with the moisture that is in the wood.
Polyurethane glues will bond wood, glass, ceramics, metal, stone and most rigid plastics.
Because the polyurethane glue reacts with the moisture in the wood, it is important to ensure that the wood has enough moisture within it to actually allow the polyurethane glue to cure.
How to make good polyurethane glue joints:
- Use a misting spray of water on one of the pieces of wood that is being joined. This will ensure that there is enough moisture to allow the polyurethane glue to cure properly.
- Apply the polyurethane glue to the other piece of wood, the one that wasn’t missed. Use a scrapper or spreader to coat the polyurethane glue over the entire surface of the wood. Unlike other adhesives where you want to make sure that you have a lot of glue on the surfaces, you only need a very thin coating of the polyurethane glue to obtain a very strong joint. Similar to applying contact cement, although you only apply the glue to one side, not both.
- Mate the two surfaces to be joined together, square the joined wood and apply clamps. One of the advantages to polyurethane glue is the working time. Polyurethane glue is a relatively slow curing adhesive; you have at least 15 minutes to properly align the pieces of wood being glued together before the adhesive begins to cure.
- Leave the pieces of wood clamped together for at least 5 hours.
- When you clamp the pieces of wood together a foam like substance will ooze from the joints. This is common with polyurethane glue and should not be wiped off until the clamps are removed. The glue that comes out of the joint is easier to remove once it has cured. Use sandpaper or a cabinet scraper. You will find that the excess glue does not become rock hard like other glues making it difficult to remove.
Shelf life of polyurethane glue
If there is one disadvantage to using polyurethane glue it is its shelf life. Depending on the environment that the glue is stored in the maximum shelf life of a container of polyurethane glue that has been opened is a maximum of six months and under certain conditions it may not be usable after as little as a month.
From an economic standpoint, as polyurethane glue is more expensive than other adhesives, it is best to purchase just enough glue for the project that you are working on.
Safety issues of polyurethane glue
Polyurethane glue contains isocyanates so it is important to work in a very well ventilated area or use a resperator when working with this adhesive.
As the polyurethane glue reacts when it is in contact with moisture it will react to the moisture on your skin. When working with polyurethane glues, safety glasses and plastic gloves are highly recommended.
Selection of polyurethane glues