I went through this when I finished my Hun. While this may not be absolutely correct all the time, what I kind of came up with was this. Most all D and F models had mostly painted wings (see attached photo). In my case the 1968 Thunderbird scheme still had natural metal fuse. When they prepped them for the 69 season part of the fuse was painted (see attached photo), but they only flew this in practice before adopting the F-4. By 1970 I would think anything that looks silver is paint. If it looks very uniform but not polished like a Thunderbird it is probably paint.