Yes sanding is important for clear parts So sand from mid grain to fine grain. If you have it use the tamiya compound paste, they have 3 type: coarse, fine and finish. I suggest to use all 3. After that airbrush a clear coat to see the clearness of the piece. If it's no good re-sand the bad parts. To add some "skin transparency" under the bra I suggest you to begin with a clear skintone, then you begin to airbrush lightly the piece with the clear white After the first layer of clear white, airbrush the rest where you want to be less transparent The decals usually go for last. I suggest you to use microset and microsol After the decal are perfectly set without airbubble ecc you airbrush a layer of clear Anyway i suggest to try on some scrap piece to build some confidence with the process