I'm replacing a bathroom sink with a ceramic drop-in model. The new sink's cut-out template calls for a 17 3/4" dia. cut-out. Unfortunately, the current cut-out is just under 18 1/2" dia. I REALLY don't want to replace the counter and cut a new opening. I was thinking of putting some kind of shims or spacers around the opening, gluing and screwing 3/8" plywood strips around the inside of the cut-out. The counter is particle board covered with formica. I doubt whether I can get the plywood to bend around the opening, so I'll probably have to cut short strips and attach them end to end. The new sink is supposed to be secured to the counter with silicone caulk.
My questions: 1) is this a terrible idea? 2) if not, is there a product specifically made for this?
Any suggestions or ideas would be appreciated. Thanks in advance for your time.

Have you tried the new sink? you are talking about 3/4" total, or 3/8 all around, which may be covered by the rim of a ceramic drop in sink. If it still won't work, I would replace the top of the cabinet and put new formica on. I personally wouldn't leave an open gap that water could seep into and destroy the countertop.

I use Poly Seam Seal to secure drop in lavatories.
If the opening is too big, replacement of the top is the only viable option as I see it.

if the sink is 18 inch round and the opening is 17 3/4 inch it might just cover. just slightly but if it will cover then shims of some sort brought up to the height of the formica will give you somewhere to glue the sink down.
as suggested use polyseamseal. apply a generous coating to cover all the shims and the formica. set the sink, clean up with a damp cloth and wait a day until using.
make sure the drain is lined up dead center because you have no play here.
AGAIN, cover everything well with caulking to make darn sure there is no chance of water getting under the rim..

Why didn't you get a "standard" 19" drop in sink which is what is usually used. 17 3/4" sinks are typically undermount or rim type.

Thanks for your reply. Yes, I've set the new sink in the opening and it does cover it, but not by much. The current cut-out is 3/8" larger all the way around than the template calls for, by that I mean that there is 3/8" less countertop around the edge for the sink to sit on. I guess I'm nervous about not following the installation specs, but I don't have the time or money to replace the whole counter top.



Edited 1 times.

I should have mentioned that I did buy a standard 19" diameter sink. The cut-out template that comes with the new sink calls for a 17 3/4" cut out. My cut-out is 18 7/16". So my options seem to be 1) replace the countertop and cut a new opening to the specifications (ugh!) 2) place shims around the present opening to bring it to the specified size, 3) try to find a good quality sink that will fit in the current opening. I'm looking but no luck so far, or 4) try to find the same sink that I'm replacing, which is a cheapo that will probably rust out quickly and I'll have to replace it again.

if the new sink covers the available opening, just put some shims or blocking inside the cut out, lather it up with a good polyseamseal caulking and shmush the new sink into the caulking. if you have sufficient caulking with no gaps for water to run under the rim, once the adhesive caulking sets, the sink is going nowhere...

The sink should cover the cut out, so just install it, then seal the rim to the counter top.

Set it in Polyseamseal and let it cure for 12-24 hours. Then caulk the countertop-to-sink joint with Polyseamseal and it will never leak or budge. Promise.

if he puts a generous bead of polyseamseal down before setting the sink, after setting and cleaning up the excess he will not need any additional caulking. in this case, more is better.

If the new sink "covers" the cutout, why bother with shims? The sink is not going to fall through the hole.

Spend the thirty bucks and buy a Formica counter and cut a correct opening.

"If the new sink "covers" the cutout, why bother with shims?"
i agree. but it depends on how much it does overhang the cut out.

Spend the thirty bucks and buy a Formica counter and cut a correct opening.

What possible difference could that make? The seal is made around the perimeter so unless the cutout shows around the sink there is NO way the sink can "fall through", especially a round sink, (isn't that why manhole covers are round)?