I had start working on the SD Gundam yesterday. The Gundam were made first by washing all the part with soap, Glo or Body wash. You can wash the part with toothbrush, bear in mind that hard toothbrush give same effect of using sand paper. Soak and sink all the part with soap and gently brush every inch of the part. After that, you can let it dried before you do the next step.
Next is you start to spray primer surfacer on it. Since this kit is small in size it would be wise for me to spray primer on the part with the sprue still attaching to it. After several minutes of priming all the part I realize several problem on doing this technique. You need to spray 360 in X and Y axis and also in Z axis. Since the parts still attach to the sprue some area were not sprayed and I only noticed after some time. It would be wise for you to have some means of holding the sprue so that you will not accidently tempered the sprayed parts.
This kit come with parts that had different color in it. Its depend on how you want to do the coloring but for me I'll priming all the parts so that I'll get the best color effect thus making it look more realistic. Owh, before I forget you must sink your spray can with warm water to get the best sprayed pressure and making the output spray smooth.
After you had done spray priming your parts you must wetsand all the parts so that the base paint will stick better to the parts. I am using 1200 grade paper sanding to perform wetsand for the parts. Water and sandpaper were used to perform wetsand. Dip your sandpaper into the water and then start sanding at the area you wants.
Please keep in mind to sand at the corner and give more attention too seamline caused by molding process also for leftover sprue.
Priming will raised all the defect that cant be seen by naked eyes. Holes, seamline and other defect can be seen clearly after priming process were done.
Wetsand all the areas and dont be afraid to make mistake during this process because this is the time for you to prepare all the surface for the base coat spray later. After you had done wetsand, wash and rinse all the parts.