The company installing my quartz countertops says that the sink I provided, a dual mount ( https://www.elkay.com/products/hddsb33229tr3.html ) is not usable and that I must provide a true undermount. I have tried researching this extensively and can't find any information about this online.
Is this really a thing? I can't for the life of me understand why they would say the dual mount is not usable.
The sink you linked to is a nice sink. I would certainly use it as a drop-in.
I get the installers question this as an undermount. Undermounts usually do not have faucet holes built into them and I can see how this could add complexity and possible issues. To add... this job goes from... mark sink (measure of course) and drill to a very complex math/aligning task. There is no lip on the front that says the edge goes here - meaning you could drill holes and they may be off by 1/8" too far or too close to the sink, either making the install a fail (gap) or at best making the sink not centered.
Also think about this. You get it installed, you drill the holes and you think everything looks great. You install faucet and you don't notice the stem is putting pressure on one of the holes in a certain direction... done. Then two weeks later you are washing dishes and sink starts pulling out (I can tell you how to install undermounts 100% fool-proof that never come out)... well its because those damn holes are meant for a drop in - actually the whole top part.
So undermounting would be pretty easy if this were laminate (wood based) countertop. I would simply glue it in, support it... and the next day drill a pilot from the bottom and go from there. Still harder than it should be but with wood based countertop, nothing is going to crack or have major issues.
Verdict - Not often I do this... but I am siding with your installers here. You want something installed that is half-baked for the application then you install it. If you want undermount, then get an undermount sink. Yes this may have the ability to be an undermount but that is not what it was made for. It is just a silly selling point to get people to buy it. My installers would probably install it but charge me an extra $100 and that's only because they do a lot of work for me.
I have used drop-in sinks for under mounting. I have even cut double drop-in sinks and under-mounted them as a single sink, in a granite counter top.
The important aspect is the flatness of the rim of the sink, and the gap between the top of the sink and the top of rim, because this is what you'll have to fill with silcone. Absence of mounting brackets is not an issue.
You need to provide sufficient support with all-round and/or brackets under the sink. If they don't do it for you, you could tell them to cut the opening and you'll install it yourself.
If the installer objects to drilling faucet holes at the precise locations provided by the sink, you can modify that sink: cut off the lip around the holes with an angle grinder and give them more tolerance.
Be careful not to bend the sink. And you need clearance from the basin. I've done it; it takes some patience. Sand it smooth to avoid injury, but it doesn't have to be pretty as it goes under the counter.
However, it should be within their skillset to drill those holes exactly as required.
If their argument is that the top of the sink is not flat enough, well, that makes little sense because it is a dual mount sink and it is designed flat enough to serve as an undermount.
