http://bbs.actoys.net/read.php?tid-2039640.html Making of gas tank cannon, shoulder pads, and the robot "wings" : http://bbs.actoys.net/read.php?tid-2041125.html
It's definitely primer, you can see some of the base plastic color on the right elbow through a scratch And primer also serves to normalize the colors, ensuring consistent colors throughout, as the base color can affect the appearance of any colors painted over them, usually when the base color and the paint color are on opposite sides of the spectrum, its what causes bleed through when the paint isn't thick enough, and generally speaking, the reason canvases are white
You need to look further into that thread : so many brilliant modifications - way more than only just the gun.
Details of other mods and new transformation process: ¡¾ºÚÆ»¹ûÖØ°õ³öÆ·¡¿Æ½Í·OPÈËÐΡ¢³µÐÍÈ«·½Î»¸ÄÔ죡-----±ØÐëÄܱäÐΣ¡£¡|±äÐνð¸Õ×ÛºÏÌÖÂÛÇø - ACÄ£ÍæÍø - ÖÐÎÄÊÀ½ç×î´óµÄÄ£ÐÍÍæ¾ßÉçÇø
Those tank guns are what scratchbuilding is all about. I know some other people have done tutorials showing this type of build, but could we maybe come up with a page of links to sites and pages like this to show everyone how it's done? I think some people get turned off by scratchbuilding when they see the final product, but once you see that its just different shapes and bits of plastic glued together, sanded and filed down to the shape you want is really all that it is. Anyone could do that with an x-acto blade, sandpaper, and 10-20$ of styrene.