you would have to sand the rails existing paint,not complety off but the sand scraches will give the new pain something to hold on to and keep the rail from chip down the road.dont use any sandpaper under 400 grit sand paper cuz you will see the sand scraches after you paint the rail just use 400 grit cuz if you use 1500grit the paint wont stick, if there are any paint chips try to feather them out with the sand paper, just run your finger over the chip and you will feal the paint edge just sand that till its smooth. when your all done sanding wipe the rail clean with a damp rag but go over it with a dry rag so the rail is dry for paint.use real light coats of paint and let it cure take your time letting it dry, lights coats and letting it dry will prevent chips.it may be a good idea if you dont do as may coats where your going to be sliding just get it coverd on top. im an auto painter so if you need anything else just PM me.