I decided to prime only what van's said needs to be primed. My question is this, if you sand on the ribs with scotch brite, do you have to prime it afterwards? I am assuming that on the ribs you can sand the whole thing and not have to worry about it. Thats what we did at A&P school when we were recovering a piper cub. Never primed it afterwards. I am now putting on the stiffeners on the rudder and did alot of sanding to get them smooth after I cut away the portion that you had to. I see it's got a shine that the ribs don't and was wondering if I sanded off the protective covering and they need to be primed now. Any ideas?