Hi, again, rube. Is the paint splitting or the wood? If it is only the paint that shows cracks, there is no need to do anything after you strip off the old paint before repainting. If the wood is splitting, then you want to fill the splits/checks with the Bondo. The chances are that water is getting into the end grain of the wood and causing the splits in the wood, but it could be that old paint underneath is not moving with changes in temperature and humidity. The only way to tell for sure is to remove the old paint. It could be too that the glue and nails are giving up at joints and the Bondo, if applied to sound wood, will act as a glue and filler. Seeing it without the paint will help you a lot.