I think that if you have rot in a few locations chances are more of the inside of the rails are rusting out and thinning the metal. I had this problem on my front rails and ended taking them right out. AMD sells them for about $350 bucks each and for what it's worth I would replace them even with a nice driver quality car you will be happier that you did. If the rot is in a small area and the rest looks solid then a fix would suffice.