There is much debate in the fine scale model forums about using a Primer or a Surfacer, and what are the differences. Simply put, no one seems to really know what the differences really are. Some brands alternate using the terms primer or surfacer leaving some people to guess that there is no real difference. After all, you can buy a spray primer and cover a surface to paint, and you can do the same thing with a spray surfacer. But there are some differences.
Typically a primer is simply to prepare a surface for painting, especially if the surface has differences in color. Use a primer to make the surface a uniformed color. Then paint over that.
A surfacer is a little different. It's used to prepare a surface -- to cover holes and seam lines: to make a uniformed surface. A surfacer can be thick and many use a rating system similar to sandpaper to indicate coarseness. For example, a surfacer with a rating of 500 will be more coarse than a surfacer with a rating of 1000. And a surfacer with a rating of 1500 will be less coarse than the 1000. The higher the number the more smooth the surface.
Use a surfacer for additional finishing, such as sanding. Let's say you have a prop with a surface that has seam lines, or even layer lines, like those from a 3D Print. To help smooth the surface sand it first, then apply a surfacer. Sand some more. Then prepare for painting with a primer.
Not all surfacers will work well with all materials. For resin use a resin surfacer. The Japanese brand "Mr. Hobby" is a great line of products for the injection model enthusiast. They produce primers and surfacers for plastics, resin, and metal. Use "Mr. Resin" for a resin surface. "Mr. Metal" for metal surfaces (such as photo-etched metal parts), and Mr. Hobby for the plastic commonly used in injection model kits. The Mr. Hobby line also has combinations, such as a "primer surfacer."
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