Water based polyurethan curing milky on doug fir pet stain

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New Member
Dec 28, 2021
Tacoma, WA
Hello all,

I have been refinishing my old fir floors in my house. They were pretty beat up and I have a pet stain on the south end of my house. I have done 3 layers of satin water based polyurethane and it dries nice and clear. The issue is that 3 days after the last layer, the poly turned milky on the pet stain. It seems to be curing milky where the joists are and near the edges of the boards.

I have 51 yr old 3/4" douglas fir tongue and groove tongue-nailed directly to the joists with no sub floor. This is in an attic space that I only keep heated during the drying process. Maybe the issue is it is not being heated while the poly is curing, only drying? The humidity of the room is between 50-55% I've been careful to stir the poly well while using it and the only spots that have dried or cured milky is the pet stain and one other stain that I'm not sure what it is.

Does anybody have any ideas of why this could be happening? Could the polyurethane be interacting with the pet stain? The stain is at least 10 years old.
Milky color is usually due to moisture coming out of the wood and getting trapped under the poly. I’m thinking you turning on the heat in a normally unheated space brought about some moisture movement within the wood itself. The attic in it’s normally unheated state would allow the wood floor to become acclimated to that state that it’s normally been in. Now you turn on the heat, that usually isn’t on, and the wood floor wants to adjust and acclimate to this new state. Moisture in the wood itself is now moving around (leaving) while the floor is trying to achieve equilibrium with its now newly heated environment. The wood floor itself is only so thick but you mentioned having issues at the joists that the wood is directly nailed to which would then allow any moisture in the joists to migrate up through to the wood floor itself and then out to the environment but now it’s trapped by a layer of poly.

@Mark Brown is full of wood floor acclimation information, maybe he has something to add.

As far as the pet stain I’m just gonna throw a stab at it. I’ve had to deal with seriously pet stained subfloors that were ‘dry’ such as yours because the pet staining was also old such as yours. When I stuck my moisture meter into the pet stained subfloor I got high moisture readings. Was it due to what I call magic piss crystals causing my meter to read high? Or was there in fact some excess moisture in the floor even though the stain was old? I couldn’t say one way or the other for sure but something definitely made my meter jump just as something is causing your poly to cloud.

Did you use a sanding sealer after sanding the floor but before applying the poly? If not don’t feel bad cus not many people do and they get away with it all the time. Regardless, that may have helped with the stain spots if it is a reaction. Don’t know 100% that it would prevent it but I am also just throwing out some what if’s and coulda beens.
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Thank you for your reply. It could be moisture coming up from the joists and around the pet stain. I'll have to sand it off on that area and apply new poly there. I will try applying the poly again there while it is not heated. It is a bit odd too, because the poly didn't cloud up until I applied my third coat.

I did use a water based sanding sealer which also clouded up on the pet stain. I sanded it down until I couldn't see the cloudiness anymore and started applying my satin poly.

I will try to leave the room conditions the same and give it another go with it being cold up there, apply very thin layers, and make sure to keep it well mixed.

The temperature here in the PNW is about 23-30 degrees currently and the room stays around 50 degrees. Should I hold off until it warms up a little? My wife is not happy with all the furniture from that room being inside our bedroom. We're currently sleeping in the guest bedroom downstairs to keep the dogs off of the floors upstairs.
I don’t know if leaving the heat off is the best answer since cold temps can lead to similar situations with poly clouding. I would think turning and leaving the heat on for a good period of time (weeks) prior to, during and after the whole finishing process would be ideal. Does that fit in with your schedule, prolly not. Maybe waiting for a better time of year when the temps are more in your favor to finish the floor is the best option. I do understand your situation and feel your pain. When I finished my floors I pushed all the furniture into the bathroom and kitchen. My wife sat on the couch in the kitchen the whole damn time just waiting for when we could put the furniture back. I wanted to strangle her.

Couple other thoughts, how much time did you allow before you applied successive coats. Cold temps will drag out cure times. The joists being in contact with the floor will affect heat transfer, as well as moisture, more so than where the joists aren’t. Sorry I don’t have an answer for you as to how to solve the problem because honestly it’s a job I would have walked away from because of the conditions you’re trying to finish the floor in. I’m sure I would have the same problems as you as well as one additional problem, getting paid.

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