It really depends on what plastic you are using. If you have all cast material (best) then you would can use #3 or 4. Those 2 are water thin and require very precise machinework. If you have some small gaps they just won't work, that is where #16 comes in, it is thick goo and will fill in the gaps. If you are using extruded, get some weldon #5, it is also water thin and requires precise joints. I use #3 to build entire skimmers all the time.