Cement or another solid pore filler suitable for ABS?
Some quick research shows me that ABS can be slightly affected by the composition of polyester resin. Automotive body repair filler, commonly called Bondo (brand name) is mostly polyester resin. I've found in my reading that some consider the "slightly affected" aspect to mean that it adheres well, others believe that the material will weaken and damage the ABS.

You could, of course, purchase some body filler and perform tests. It sands well and would certainly remove the layer lines.

If you determine that such material is not a good solution or would rather not take the gamble or perform the tests, you have an option with another product called microballoons or microspheres. It is a product that has an accurate description. The material is hollow spheres of glass of such tiny dimensions that they will blow away if you breathe on them.

The primary purpose of this product is to fill gaps, create fillets and similar structures without adding appreciable weight. One would use them with thin epoxy resin and mix to desired consistency. Some descriptions include "mayonnaise or peanut butter" depending on the application.

With both of these products, easily sanded is also in the description, although your choice of epoxy will also contribute to that aspect. Because the microballoons are mostly air, they do sand quite cleanly when mixed to heavy or thick consistency.