Don't try to sand the seams on a fabric-covered plane without a layer of paint over them. You'll make a frayed mess that no amount of paint will hide. Once you've layed down some paint, you can wet sand the seams being careful not to cut into the fabric. Several rounds of paint/wet sand will help smooth out and minimize the seams.

There are 2 types of pinking tape that I've used; Pink-It (dbalsa.com) makes iron-on tapes from Solartex and F&M Enterprises (Stits.com) makes tapes non-adhesive tapes that are put down with their Poly-Tone paint products. I've used both and each has its pros and cons. Either will work if you're using something like dope or poly tone paints. If you're using automotive or latex paints, I'd use the Pink It tapes. There is also a UK source of pinking tape but I can't remember the site and non-scale pinking tape that you can get at the drug store. You might get some ideas from the following article: https://www.modelairplanenews.com/ho...and-rib-tapes/