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Two days ago we re-caulked our tub using DAP Kwik Seal Ultra and left it undisturbed until today. Unfortunately it developed these brownish yellow spots and I am unsure why. I haven't been able to find anything online that is similar to this situation as most articles discuss caulk that has rotted or molded over longer periods.

Despite caulking the entire tub at around the same time, the spots only appear on the seal on the outside of the tub, between the tub itself and the walls and ground. There is a fairly large gap between the tub and floor, and also between the tub and one wall, so backer rod and fairly thick lines of caulk was used in these parts.

What I am wondering is what caused these spots to appear, how to prevent this from happening in the future, and how to remedy the current situation.tub/floor seal

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    Is it a steel or cast-iron tub? Were there any chips in the surface finish? I have to wonder if it's rust... – Ecnerwal May 7 at 13:04
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    Yeah, it does look like rust. maybe from nail heads behind where the caulk is run? DAP caulk is water based, so until it dries it can cause rust if there are steel nails or brads underneath it. You might have to remove is and start over, with a silicone based caulking. – J. Raefield May 7 at 17:26
  • @Ecnerwal I believe it is just an acrylic tub, and yes there may be some of the finish chipped away in parts, I'll have check if they are around the rust spots – dan333 May 7 at 17:53
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    @J.Raefield I was afraid that might be the answer but I may go ahead and do that, as it does sound like these spots are probably rust. – dan333 May 7 at 18:07
  • If it's an acrylic or fiberglass (or plastic in general) tub, J.Raefield probably has the right answer with nails or similar non-tub sources of rust. Finish chips in plastic don't expose metal as they do on iron/steel tubs. – Ecnerwal May 7 at 18:46

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