Building models with a domain specific language enables targeting specific platform and framework functionality. We built a domain specific language for use in modeling applications targeting our business application framework. Such models are used for tasks including generating C# code and producing object-relational mappings for business objects. The paper briefly describes the framework and its accompanying domain specific language and then describes issues we encountered in using an unconstrained UML tool to express our models, solutions we developed to deal with those issues and observations about the suitability of UML for application to such problems. We found that making a general-purpose, extensible modeling language serve the needs of a targeted domain specific language is a lot of work and is only partially successful. We conclude that what is needed is a more general purpose framework for creating domain specific languages and tools for them.