An apartment building that provides affordable housing for 25 seniors and a home for a preschool will get much-needed renovations through a $3.2 million tax-exempt bond issued by the Massachusetts Development Finance Agency. The proceeds will pay for exterior masonry repairs, basement waterproofing, and replacement of windows, kitchen cabinets, flooring, and interior walls in the 78-year-old St. Joseph Hall apartments on Rosary Drive , officials said in a statement. MassDevelopment also helped obtain federal low-income housing tax credits, which provided about $1.4 million in equity for the project, officials said. The Watertown-based nonprofit Metro West Collaborative Development is the project’s sponsor, officials said. The St. Joseph Hall units are set aside for seniors earning up to 60 percent of the area’s median income; the renovations will help ensure the units remain affordable, officials said.