58er,
I'm not a pond builder but If this is a new pond then I'd be asking those questions of the person you paid to dig it. It sounds as if the sand/clay boundary needed several layers of compacted clay, properly done, to seal. If this person was not experienced but rather was just a dirt mover, they may not know this or may not stand behind their work. But after reading lots of stories like yours on this forum the word of wisdom always seems to be, not so costly to do it right the first time, hugely expensive to try to fix it later.
This is probably a tough time of year in MI to be draining, drying and trying to repack clay into your pond. But the pond digger really owes it to you to get it right. It seems you have lots of blue clay onsite so you could get the proper sheepsfoot compactor, dig a hole to produce a supply of clay and start packing it properly but not before you discuss who is paying for what.
I would describe the polymer sealants as useful 'band-aids' that may slow it down a fair amount but no one really knows how long that band-aid works either and the further you get into it, the more frustrated you are, the more you wish you didn't have to pump water into it, and the more you wish you had got it fixed right, right away.
Oh, and it sure is good to see more pond owners from MI join. Tell us more about your pond, location (woods, fields, groundwater vs with a dam and a watershed) and what your goals are for the pond?
Pictures would be great too!
