To Seal Fenders to Wall ... or not?

I am almost ready to mount aluminum fenders to my sandwich foam/ply walls. I have locations noted where I have solid wood, to receive threaded inserts.
I am contemplating attaching the fenders such that there is a small gap between the fender and wall, through the use of 1/8 inch thick plastic washer. This should allow drainage and no water accumulated.
Has anyone tried this technique and have feedback on its success?
My experience with creating ease of drainage between wood parts on my cedar deck that I designed and built 25 years ago, and the success of this approach, has steered me to seriously contemplate this option.
Any other ideas or are people just laying down lots of caulking?
I am contemplating attaching the fenders such that there is a small gap between the fender and wall, through the use of 1/8 inch thick plastic washer. This should allow drainage and no water accumulated.
Has anyone tried this technique and have feedback on its success?
My experience with creating ease of drainage between wood parts on my cedar deck that I designed and built 25 years ago, and the success of this approach, has steered me to seriously contemplate this option.
Any other ideas or are people just laying down lots of caulking?