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Is PU (Polyurethane) Adhesive waterproof?

A simple guide as to whether a PU adhesive is waterproof, which the best PU adhesives are, and what they can bond.

There are many polyurethane adhesives on the market today and it can be confusing to work out which one you might need for your application. Probably the most common application for polyurethane adhesive is for bonding wood, particularly for applications which require a high level of water resistance. But polyurethane adhesives, in different compositions, can be used for bonding many, many substrates including metals, fibreglass, some plastics and rubbers, concrete, stone and brick.

But how waterproof is polyurethane adhesive? This is a good question and as always it depends! As a general rule, they do have a high level of water resistance, but it does vary on the composition type and for what purpose the adhesive has been developed.

The common single part polyurethane adhesives, like Aro-Bond® 944 and Aro-Bond® 947, comply with the D4 class of the EN204: 2001 standard, “Classification of thermoplastic wood adhesives for non-structural applications”. The description of the D4 class is;

“D4 = Interior with frequent long-term exposure to running or condensed water. Exterior exposed to weather but with protection by an adequate surface coating.”

So the short answer is yes it is water proof as long as adequate surface coatings have been applied. These products are often used in window, door and external joinery and woodworking applications and they give a very high strength and weather resistant bond.

These wood bonding polyurethane adhesives are also known as moisture cured polyurethane adhesives (or known by the acronym MCPU) as they require moisture to cure. This moisture will either come from the substrate or atmosphere, so in normal situations they do need at least one of the substrates to be porous. If both substrates are non-porous, you can add moisture using an over mist of water and then should kick off the curing process.

If you would like further information on our range of polyurethane adhesives, please contact us on 0117 971 1364 or email us on