What is the difference between a D3 and D4 PVA wood glue?
Water resistance. Both D3 and D4 PVA’s will give a water resistant bond, but D4 is the higher standard, meaning that it can be used for external joinery. D3 adhesives have been well established for internal, general purpose assembly and can also be used for laminating and veneering as well. They are simple to use, clean up easily whilst wet and are fast setting. Most products you can buy will be D3 standard adhesives. General purpose PVA, such as those supplied for dilution and as suitable admixtures will be D1 standard and have very little resistance to water and humidity.
D4 standard PVA adhesives have the highest degree of water resistance and can be used for external joinery applications. The D4 standard actually includes a 6 hour boil test to really put these products through their paces, so if you think that your project needs this extra performance, pick up some Prowood D4. It is important that joints are protected to ensure their longevity and make sure they are a tight as possible. This makes for a strong joint, one in fact that will be stronger than the wood itself.