White oak stained black from caulk?

[Andrew Goraj]

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Hey guys, quite the quandary. I made some R/QS white oak counters. Finished with 3 coats of general finish high performance poly. The Inside edge (can’t see it now since the sink is installed) where the cutout is for the sink is finished with 6 coats. I used GE silicone 2 kitchen and bath for mounting the grade 304 stainless sink to the countertop. Before I even turned the water on, the countertop began to exhibit this black staining. Within about 12-24 hours it started to appear from the time I caulked the sink in. The staining is only seen on the sides of the sink, not the front or back, so it makes me think it has something to do with the end grain soaking up the caulk and having a chemical reaction with the tannins in the oak. I’m real bummed as I used a lot of good material and the rest of the tops all look great. My only thought was to take the sink out, sand the counter back down, use some oxalic acid to try and remove the staining, refinish and next time mount the sink with a less volatile compound like latex based caulk? Has anyone ever seen this before? Any experience here is appreciated! I don’t mind trying again, but not sure if it’s in vain. Thanks, photo should be attached

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[John TenEyck]

I'm guessing the acetic acid (I think) in the caulk ate through the HP Poly and then reacted with the oak. You can proceed as you described but I think if you are going to go the effort of removing the stain I would strip the HP Poly off the entire countertop and use something more durable. Waterlox or Arm-R-Seal would be better choices. If you want to use a WB product, then consider Target Coatings EM-8000CV plus cross linker. All those products will need at least 3 or 4 weeks to fully cure.

John

[Andrew Goraj]

I'm guessing the acetic acid (I think) in the caulk ate through the HP Poly and then reacted with the oak. You can proceed as you described but I think if you are going to go the effort of removing the stain I would strip the HP Poly off the entire countertop and use something more durable. Waterlox or Arm-R-Seal would be better choices. If you want to use a WB product, then consider Target Coatings EM-8000CV plus cross linker. All those products will need at least 3 or 4 weeks to fully cure.

John

Funny, I actually had used arm r seal first, as it’s my favorite finish by far, but did not like the ambering. Normally I prefer it, but for counters we decided the non ambering water based looked nicer. So, I already sanded down and refinished these once so far. Just one of those projects I guess.

[johnny means]

The staining is from the metal of the sink.

[andy bessette]

Now that it is damaged I would edge band the cutout with solid wood and epoxy seal it before caulking. Also I would use an absolutely waterproof finish.

[Andrew Goraj]

The staining is from the metal of the sink.

Well, I think you might be correct. I did a test on the unfinished cutoffs of the countertop with the same caulk. Smothered the caulk on the end grain of the oak cutoffs and a bit on the faces, no staining.

I’m now finishing a bunch of the cutoffs with the same method as the counter, and seeing if having the water based finish applied and then introducing the caulk results in staining.

My guess is though at this point, having finish on the oak before applying caulk won’t make a difference. And it’s the iron in the stainless steel sink that combined with the caulk to cause the issue. I’m still perplexed why it is only on the sides / end grain of the counter, wouldn’t the front and back also show staining?

[Bob Cooper]

I have a 2.5” maple counter with an undercount copper sink. It’s a round sink so plenty of endgrain. I know people
freak out about having exposed endgrain like this but I have had no issues. So one thing you might consider is buying a little bit bigger under mount stainless sink, cut out the new larger hole and mounting from below. The only finish even on the cut out is mineral oil…I don’t even recall if I coated that endgrain with anything. If I did it would have only been oil based poly

[andy bessette]

I have a 2.5” maple counter...

Pretty sure maple will handle this much better than white oak.

[Andrew Goraj]

I have a 2.5” maple counter with an undercount copper sink. It’s a round sink so plenty of endgrain. I know people
freak out about having exposed endgrain like this but I have had no issues. So one thing you might consider is buying a little bit bigger under mount stainless sink, cut out the new larger hole and mounting from below. The only finish even on the cut out is mineral oil…I don’t even recall if I coated that endgrain with anything. If I did it would have only been oil based poly

I thought about this. Part of the problem is I laminated the outer edge of the counter to look like 1.5” thick. But the material is actually only 3/4”. So, if I do an undermount the exposed sink edge would reveal the 3/4” thick oak. I’m not sure how this would look.

My other thought if it is indeed the iron in the sink causing my issue, is doing a slightly wider top mount sink made from some composite granite type material. But it’s black, and the wife prefers stainless.

I spent weeks making these counters, and a lot of wood. Plus granite counters are over $5,000, adding insult to injury. Really trying to come up with a solution but every turn presents a new obstacle!

[Rob Luter]

The staining is from the metal of the sink.

^ This ^

The acetic acid created by the silicone curing process just exacerbated the situation if anything. The Oak is somewhat acidic anyway and will react to the steel. Even Stainless Steel.

[Andrew Goraj]

I decided to go with a slightly larger sink made of fireclay (no iron in it). This is Probably outside of most peoples wheelhouses, but anyone know of a safe caulk to use this time around? Maybe just 100% pure silicone like used in aquariums, or something latex base?

[andy bessette]

I would use something like West Marine's Multi-Caulk.
https://www.westmarine.com/west-mari...7_003_502.html

Or 3M 4000UV.
https://www.amazon.com/3M-Marine-Adh...000XBDJWC?th=1

[johnny means]

Well, I think you might be correct. I did a test on the unfinished cutoffs of the countertop with the same caulk. Smothered the caulk on the end grain of the oak cutoffs and a bit on the faces, no staining.

I’m now finishing a bunch of the cutoffs with the same method as the counter, and seeing if having the water based finish applied and then introducing the caulk results in staining.

My guess is though at this point, having finish on the oak before applying caulk won’t make a difference. And it’s the iron in the stainless steel sink that combined with the caulk to cause the issue. I’m still perplexed why it is only on the sides / end grain of the counter, wouldn’t the front and back also show staining?

The endgrain is wicking up the metal/water concoction created by the caulk. It's common with oak and will happen with glues, epoxies, and finishes. I've seen epoxy wick it's way three inches into a solid piece of oak. To avoid this issue, seal your end grain with something fast drying and clear. I like rattle can lacquer.