As mentioned in a previous post on tank liners and weirs, building a tank weir into a tank liner is not the best method for handling this type of feature. Fabricating the weir is an intricate process and usually needs to be done with a thinner gauge of material. By default, this makes the weir the weakest part of the liner because it’s thinner. Furthermore, all of the angles can create a stress point that could cause the liner to rip or tear at the weir. Finally, the material underneath the weir can pull away from the tank wall causing a poor fit of the liner.
A preferred method is to remove the weir from the tank and grind the tank wall smooth. Install the liner as normal and place a bulkhead fitting at the weir hole. From the bulkhead fitting insert a piece of PVC pipe with a cap on the end. Cut some slots in the pipe to allow solution to flow into the pipe. This rigid pipe setup now acts just the same as the original weir. Besides eliminating all of the concerns of fabricating a weir into the liner, the reusable rigid pipe setup also eliminates the cost of having to fabricate the weir into the liner. For the price of some pipe and a bulkhead fitting you end up with a better fitting and longer lasting tank liner.