Kel Seal Elastomeric Custom Patch. This is similar to caulking from a tube but in a tube that you can used a spoon or putty knife to dig out and spread. It comes in Textured with plastic sand mixed in or smooth. It also comes in brushable and trowable. I use the Trowable as it is a bit stiffer to work with and does not penetrate surfaces as fast as the brushable, giving me a bit more time to move it around. The textured gives a nice arid ground base for fast painting and dry brushing.
It comes in quarts and gallon containers, this brand I purchase from Kelly Moore Paint Stores. Other stores have their own brand name items that do the same thing, just tell them your looking for caulking in a tub to brush or trowel on and they will take you to their version I am sure.