If you're in need of lap sealant it doesn't get much better then Firestone's Lap Sealant.
Firestone Lap Sealant is designed to seal and mechanically protect the exposed cut edge of QuickSeam Products. Lap Sealant is also used as a general purpose sealant in other applications as specified by current Firestone specifications and details.
The sealant is applied with a mastic gun at the specified location using the plastic nozzle applicator supplied by Firestone, being sure to keep the nozzle applicator centered over the lap step-off. Take care to leave a mound of Lap Sealant directly over the splice edge.
Flammable - Keep away from sources of ignition. Do not smoke when using. For professional use only. Use only in well-ventilated areas. Avoid contact with skin and eyes. Refer to Material Safety Data Sheets. Keep out of the reach of children. Recommended cleaner is rubbing alcohol followed by soap and water.
Solvents: Light aliphatic solvent
Solids (%): Min. 80
Specific Gravity: 1.34 - 1.46
Flash Point: 11
View Firestone Lap Sealant Application Video
This lap sealant saved me twice. I had installed a new EDPM liner and there were a few places where I was just adding a strip along the bank as I was shy liner for underneath the flagstone bank. Most of the time, you lay out the liner on a very large flat surface to seam with seaming tape, or you put something flat underneath so that you can make a perfect seam. However, I had two places where neither could be done. Enter Firestone lap sealant. A few seams had wrinkles which is a potential leak point. No worries, I filled the wrinkle with the lap sealant in those situations - especially on places where the wrinkle moved depending on how I lifted the liner. The other places were along the flat seams where the liner met the layer beneath. The tape is always just beyond about 1/4". I ran a bead in this place and smoothed it to have an insurance layer sealing the two edges into one. That way, no water could get to the tape, no spaces were left, and dirt or other debris couldn't make its way in. Beware that the sealant is very messy (where gloves) and takes a bit of time to set up. Afterward it feels like a soft rubber and nearly impossible to remove.
When seaming, you may not always have the advantage of laying the liner out on a large flat surface making a perfect seam. I used this as extra protection after priming and tapig, especially in areas where there were wrinkles and gaps to ensure no water could *********.
Exactly what I needed to stop a leak I had between the skimmer and the liner. Wish I would've just used this to begin with. I had extra so used the leftovers to seal around some rock in the waterfall which has also helped with the flow