I think Martins told you all you need to know tbh. As regards what type of treatment, well anythings better than nothing! One thing I would say, is don't be tempted to try and fit panels to cover in the whole area, as no doubt some will advocate. In my opinion, this simply exacerbates the problem, as you'll never stop moisture getting in there, and by doing this, you are sealing it in to fester away quite happily! Far better to rinse it out regularly, if you have a compressor, fit a length of air pipe to the end of your air gun, to blast out crap and moisture, then squirt whatever takes your fancy back in. Oz.......