I use a round sole spokeshave to plane concave surfaces (i.e. the sheer clamp on my S&G boat). There are a number of other tools for single and double radius surfaces. Below is one from Woodcraft. Japan Woodworker has some interesting wooden planes (single and double radius)... You could also use a curved scraper or improvise by wrapping sandpaper around a tennis ball, etc. I personally like this little bronze number.