Filling trim holes

Q: How can I fill nail holes in trim? During a recent renovation our contractor used MDF trim and baseboard, and even though we filled the nail holes with wood filler before painting, they still show up as a small indentation. What did we do wrong and how can we fix it?

A: Medium-density fiberboard (MDF) is used for more and more paint-grade trim and it's an excellent material for this application. But like any surface, filling nail holes involves more than meets the eye. You could have avoided the problems of filler shrinkage by using a product that hardens by chemical reaction, instead of evaporation. Any kind of filler that dries when you put it on gets smaller in the process. Most wood fillers fall into this category and you really need to fill, sand, and then fill again to get level coverage. Sometimes even three applications are required. A better option is auto body compound. This hardens by chemical reaction between the filler paste and a hardener that's added immediately before use. One application covers nail holes without forming any indentation. At this stage your best approach for repair is to sand the nail hole areas with 120-grit paper until the paint is dull, and any old filler is level with the surrounding MDF. Next, fill remaining dimples with auto body compound. It's very inexpensive and comes in several formulations. Be sure to choose the smooth type that’s free of glass reinforcing fibres. Paint the entire trim sections again to cover the filler.