I have been using Tamiya's Liquid Surface Primer in a rather non-standard way. I have found that the solid material that settles to the bottom of the bottle when left undisturbed makes an excellent fine-textured putty that dries very quickly and is very durable.
In order to get to the material without diluting it with the liquid that rises to the top of the bottle I fashioned a tool similar to a soda straw (see left-hand side of photo) out of a length of plastic tube and a square piece of plastic sheet. The length of the plastic tube should be long enough to penetrate the material in the bottom of the container but not hit the bottom.
Prior to inserting the tube in the Liquid Surface Primer container I place my finger over the top of it and hold it there while inserting the tube. This causes the liquid to be displaced by the air that is trapped in the tube when I inserted it (see the right-hand side of the photo). The plastic sheet holds the tube in place.
Now I can insert a paint brush down the tube into the solid material at the bottom of the tube and extract a quantity of it undiluted but still in a slightly liquid state. This ability to apply putty with a paint brush allows one to position just the right amount of putty very accurately. It will can be sanded in 15-30 minutes depending on how many applications are needed to fill the space.