Counter top tile repair ...

I am in the midst of a kitchen counter top replacement of granite tiles in the vicinity of the sink. I've removed the 4 [2 on each side] adjacent 12"X12" tiles and the smaller front and back tile strips.
The tiles were laid on 3/4" wood underlayment [softwood core with wood veneer on each side]. Some rot has begun in the underlayment. I will need to replace about a 2'X4' section of it. The reinstalled tiles will overlap the joints.



I cannot find a local source for underlayment of that thickness.
Is there any reason I should not substitute 3/4" plywood for the underlayment material?

How about coating the new wood with epoxy to stop any more rot from developing?

How about marine grade plywood? My choice would be to install 1/2" thick plywood, then top that with 1/4" Wonderboard lite, glued and screwed of course...

Havasu's approach would work nicely. Your plywood is the typical underlayment. The Wonderboard is your "tile backer" which can be used for your sink repair and the backsplash.

1/2" is to thin for a counter top or for under tile.

I agree, in the best world, you use 3/4 ply with 1/2" concrete backer board. That was what I used at my house for the same granite tiles and worked great. Because we are dealing with matching heights, that is the best I could come up with. All I could recommend is to support the underside to stop the deflection.

Thanks guys for your input.
The present tile installation has been in use for about 8 years.
The tile installation has been good.
The problem is with the grout.
They grouted between the countertop and the backsplash and the wood edging in stead of using a caulk in those areas.
The grout has failed in those two locations.
Rot has set into the wood edging and the underlayment adjacent to the edging by the sink's most heavily used section.
LFAs [little fire ants] have setup housekeeping in the rot area.
At M'Lady's insistence, this project began as an ant hunt!
There has been no problem with the tiles other than grout being used where caulk should have been the material of choice.
Now, the repair of the rot area has enlarged the job a bit.

I am leery of trying to match the height of the underlayment board with multiple layers of dissimilar materials, then having tiles span a change in the underlayment.
I'm only replacing a 2' X 4' section.
The 3/4" ply should be stiffer than the existing wood underlayment board.
The only difference I can see is that the plywood's "A" surface is a bit rougher than the surface of the existing board. I probably could run a belt sander lightly over it to smooth it out some and fair the edges together.
Keeping the surface flat and consistent where the tiles overlap the seam is my greatest concern.
I agree that the best installation method would be with the concrete type board if I was completely re-tiling the countertop.
So far, I do not see where anyone has given me a reason not to substitute ply for the original underlayment board.
I'm still open to thoughts on the subject.

I would just replace the plywood but before you grout mark out what the sink will cover and caulk those lines. Sealing all the plywood with Redgard will protect it from water even if things leak.

OK, I've, got the new tiles set.
The plywood was not as stiff as the underlayment board. So, I glued and screwed a board overlapping the joint on the LH side with the RH side over and screwed into the sidewall of the cupboard below the sink. The sink will stiffen the central portion of the plywood when it's clamped into place. I looked at Redgard. But, it is only available in a gallon for $55. Too much and too expensive for a 4sqft repair job. So, I did not moisture proof the ply. But, I will caulk behind, in front of and under the sink, along the wood edging around the entire counter top, and the joint between the counter and the backsplash with a sanded caulk to match the grout. Finally, the sink will be caulked under its lip to the tiles when it is reinstalled. Tomorrow, I will begin removing the grout from the rest of the counter top in order to re-grout the entire kitchen counter. Then, I'll reseal the entire counter top. It should end up looking almost new.
M'Lady's gonna like it when I'm done.

EDIT:
Question
When I begin to re-grout the tiles, should I caulk then grout, or grout then caulk?
I'm thinking caulk then grout after the caulking of the backsplash, the wood edging and under the sink has set up.
Then after thinking about it a bit the other way around seem to be the better way.

Re-Edit:
Grout then caulk seems to be the preferred order. And, it makes sense. The caulk will make a better seal against the grout than the grout will against the setup caulk.

I just caulked the grout lines under where the sink will hide it and then grouted. Then my sink was to big, couldn't get the clamps and bolts in under so I stuck it down with silicone. No water is ever getting under my sink. The are other products like redgard and I think some come in quarts but you know how that pricing goes. Mine is just on plywood too, I figured the colour will be wrong about the same time as there are problems with it.