What is your old liner made out of? Barring something bizarre like exposure to raw oxygen ions in space, EPDM does not become "porous." I wouldn't really expect that of even an old PVC liner either, although something like polyethylene (which I think was used briefly 20 years ago) could well become porous.
However, as with roofs, leaking almost never occurs in the broad areas of the pond unless there has been mechanical damage (tree roots for example). Leaks occur at edges and interfaces. The most likely suspects would be your bottom drain (if you have one), skimmer / edge drain, pump plumbing, sharp creases in the old liner which have cracked (I would think likely with PVC and PE, but not EPDM), or UV deterioration along the top edge. Since my reading of your note is that you lose a lot of water, the latter problem is probably not it, since it would stop as soon as the water dropped below the top edge where the leak was.
I would check these areas after draining the pond before getting more elaborate in your repairs.
A regular (45+ mil EPDM) liner placed over the old one should do the job handily. As mentioned above you would want to punch many holes in the old liner so that water trapped between them will drain out rather than stagnate.
Polyureas and polyurethanes are definitely NOT a DIY project.
These products, while pricey, have substantial technical benefits. However, being a 2-part reaction system they must be installed by professionals using precision equipment:
- a 49:51 mixing ratio is NOT the same as a 50:50 ratio, and the properties of the 49:51 coat will vary significantly from nominal. Good metering equipment is expensive but absolutely mandatory.
- you need to have a fairly heavy compressor (100+ cfm @ 100 psi or so) and use mildly expensive (~$10K) spray equipment
- as seen on the second website, you generally want to be wearing a full bunny suit and NIOSH carbon-filter respirator since the unreacted monomers in both Part A and Part B are between mildly hazardous and highly toxic, especially if inhaled during a spray application
If you do have this done for you, the resulting product should give you exceptional service. Follow the professionals' advice on surface preparation. I wouldn't think that you would need to punch holes in the old membrane since the new coating should form a fully-adhered layer on all of the surface of the old membrane. They may want you to cut out the folds and overlaps just to give a more uniform surface.
