The present invention provides an improved multi-layer composite of two or more polymeric layers at least one of which is formed from a thermosetting composition. The composite includes at least a first polymeric layer formed on a substrate and a second polymeric layer over the first polymeric layer, wherein in the absence of a boron-containing compound, the first and second polymeric layers have poor interlayer adhesion. The improvement resides in the inclusion of at least one boron-containing compound in one or both of the first and second polymeric layers in an amount sufficient to improve the interlayer adhesion between the first and second polymeric layers. Also provided is an improved curable coating composition used to form a multi-layer composite coating of two or more cured coating layers, at least one of which is formed from the thermosetting composition. Related methods and coated substrates are also provided.