To finish off the part, you then apply several coats of automotive spray filler primer (with sanding in-between each application). The results will be a very smooth surface that does not look like a 3D print.
If you want to get a smooth result from your FDM prints (Fused Deposition Modeling), you need to smooth out all of the layers that can be seen in your final print. To achieve this, Bob (of I Like to Make Stuff) recommends first trowelling on a product like Bondo Glazing and Spot Putty. This is applied in a series of layers that are then sanded until you get a reasonably smooth surface.