Are you simnply joining the two preforms with a piece of EDPM liner
material? I assume they are both set at orclose to the same level.

If I was going to join two preforms as such I wuld look and find a
piece of old preform or a piece of polyethyene plastic and cut it into
a strip approx 1inch wide. You will need enough strip material to
start at the top of one side down and around and up the other side of
each preform. I don;t know how hard EDPM liner material is to stretch
or form but fit every thing up dry first. Use a couple of small stove
bolts of #8 or 10 diam to hold all in place while you get the holes
drilled. Layt in the liner, afainst the preforms interior, and lay one
of the precut 1" wide strips over it, drill a hole though strip, liner
and preform, and secure with fastener (temporary). Continue working
down and across and up the other side drilling a hole and stretching
and fitting the strup and liner into place, and secure with more temp
fasteners. Its gonna get old fast installing and removing these
fasteners but its the only way your going to keep things aligned..
Unless you know someone who works sheetmetal or aircraft, you may be
able to borrow some Kleco's to use instead of having to install and
remove the fasteners, as it will go much quicker with Klecos.......so
if you have gotten along this far in life without em and don;t know
what they are, they certainly are not cost effective to buy just for
this project, as they run about $4.00 each plus about $30 for the tool
to use them............and your gonna need quite a few.

Depending on how tight and snug you can get the strip and liner
material to lay in the preforms cutout, will determine how many
fasteners you maya need to install overall. Once one preform section
is done, do the same to the other........I would suspectr its gonna be
hard and you will have some bulges or wavey sections but its
doable.....Just guessing I would suspect your gonna need a fastener at
least every inch or so just to pull things tight and hold things in
place.

Once both sides are fit and drilled with the strip and liner, remove
all fasteners. Make sure you mark eachpiece for proper orientation for
reassembly. Clan both sides of the liner well, and clean and scuff up
surface of preform and strip that contacts the liner. Apply a liberal
bead of RTV of thickness sufficient width or make two beads in the
area the liner is going to lay on, under the strip, and on the
preform, you can also smear a thickness of RTV on both sides of liner,
and then start to install the permanent fasteners your going to use.
Snug everything down, and sort of fillet what RTV oozes out between
the liner / stip and preform with a damp finger.
Make sure RTV oozes out from the entire length of the strip and
liner .

I would use stainless steel flat head #8 machine screws, with a #8
stainless steel finishing washer under the head of the flat head
machine screw. Use a stainless flat #8 washer on the back side
(towards dirt) and secure with a stainless nut. You can get all these
types of stainless fasteners at Home Depot or a marina.......
Let it all setup at least a full 24 hours before filling with water.

I would suspect if its done properly it will hold water and should
work fine. It should last along time, if the soil and ponds are not
disturbed to put additional stress on the pieced in section.

Hope this helps. Its the way I would do it if I was doing such a
project.

Regards

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