What is the best technique for creating a knockout for an undermount sink for a concrete countertop form?

Question

What is the best technique for creating a knockout for an undermount sink for a concrete countertop form? I’m in the process of building the form.

I have an undermount porcelain sink that I want to use with a concrete countertop. I have a paper template for the sink, but it seems like it may be a little tricky to get a really clean cut in a piece of foam board that will look nice. All I can think of is to use a jigsaw and follow the template (attached to the foamboard) as best as I can. Alternatively, I'm thinking that maybe there's a way to use spray on foam to the actual sink to build a mold. I’m thinking the ID of the sink is exactly the size of the knockout, so the concrete would be flush with the edge of the sink.

looking for ideas! thx

Answer 1

a little tricky to get a really clean cut in a piece of foam board that will look nice. All I can think of is to use a jigsaw and follow the template (attached to the foamboard) as best as I can.

You can do better than that. Foam files or sands easily, but MDF might be a better (and perhaps less expensive, though who knows this year?) base material, and then you can use auto-body-filler on the edges to help get them perfect (after sawing, rasping or filing, and sanding (the first time) to size, and if you overdo the sawing the filler will fill those spots. Then file and sand/polish the filler until it's exactly what you want your sink-hole to look like, in reverse.

If it's a true undermount (thus, fully below the counter), I'm not seeing any way that spray-foaming the sink makes sense. The sink goes under, a hole in the counter smaller than the outer rim of the sink lines up with the hole in the sink, the sink rim seals to the underside of the counter...

If it is one that mounts more in than under you could use body filler to make a quick mold of the outer rim, and then use that mold to make an exact replica of the sink shape.