Hi, I have treated the underside of the saddle once when I first bought it, but since then I only treat the top with a very small amount yearly. The underside is rough and takes up much more proofide than the top. My thought was that treating the underside as often as the topside would hasten the degradation of the saddle. I can nicely buff the topside after a thin layer of proofide. Reminds me of doing same to my shoes when I was a kid. However, the underside cannot be buffed and it soaks in more deeply.

It is true when riding in heavy rain the water goes everywhere even with fenders, and to the underneath of the saddle, with the top covered. So it seems wise as you suggest to reapply also to bottom. I appreciate your comments, thanks.