Recently I picked up some Squadron White modelling putty to test out a new small gap and seam line filler in place of my old method, the milliput wash.
The Squadron putty comes in both green and white form in an oil paint style tube. I opted for white as it had been given better reviews on some military modelling forums.
The putty can be used straight from the tube, though it's a little more difficult to apply and will be a bit more grainy. Instead I choose to mix it with some Acetone to create a goopy paste that can be spatulad onto the required areas. It's worth pointing out that acetone can be pretty nasty stuff, so take care when using it and work in a well ventilated area.
Also - Don't try this method on plastics - the acetone will melt them!
Resin, metal and sculpted putty surfaces should be fine though based on my own experiences.
Once the mix is applied (you don't have to be too careful), leave it to dry for a few hours until it is solid and dry right the way through.
When dry, you should be left with a rough blob on the area you want to fill (as above - I forgot to take a pic of the gap but it was basically a hollow formed by the casting vents). This can then be sanded away using some fine grit wet/dry paper until all that is left is the flush filled gap.
Simples! A nice smooth, gap free surface achieved with the minimum of effort!