Help on how to paint your skis ?

sand the topsheet, spray paint over it using painters tape to get some solid patterns going.

it really depends on what type of topsheet you've got, encapsulated or sublimated. encapsulated graphics are printed onto paper, cloth or similar material and put under a clear top sheet, or a clear lacquer on the topsheet. this leaves the graphics underneath or inside the topsheet. the materials and inks used are chosen carefully, otherwise they can affect the bond between the topsheet and the composite layer underneath. sublimated graphics are fused into the materials that make the topsheet using special inks, suitable plastics and heat. this leaves the graphics as part of the topsheet, with the colours going all the way through the material so that even if the topsheet gets scratched the graphics will still be there.

sand then paint, if you can find it, get a spraycan clear coat, it will protect it from weather and seal it, it will also make it look nice and glossy

It's gonna chip/scratch regardless, sanding isn't going to do anything. Most topsheets have a plastic cap which will hold paint fine until you ski.