I'm using Stairtek retreads to resurface previously carpeted stairs. Some current treads have some sag in the middle, with the worst of which being 3/16". Is there a viable way to level that so that the 5/8" thick retreads don't have the additional flex to deal with? I'm considering something like Bondo putty. I'm not very familiar with most such products and assume some like Durham's Rock Hard would tend to break up too easily. Is something like Bondo or Plastic Wood a reasonable application here?
Please note, my preference would have been to replace the actual treads, but let's say retreads are part of the parameters of the project.