you can use #4. Tap Plastics has all of them. #3,#4,#16,#40 (2-part). 16 is really thick and can make a mess if you dont becareful. For cracks in skimmers I would you #4 for the actual crack so it can get in there. Then I would go over the crack with #16 inside and outside of the skimmer. I had to do this to a used one I bought from a buddy. They can be shipped, but have to be shipped via ground service. If you have any questions shoot me a PM. Hope i helped.

Ram