Stain / Poly trouble. Any ideas?

[alamode]

I recently "finished" a small project with three coats of stain topped off with three coats of polyurethane. This is basic stuff, Oak faced plywood sanded down smooth, Minwax wood stain and Minwax "Fast Drying" poly. 24 hours between coats.


After the first coat of poly dried awhile, an area developed under (or within?) the poly that has a milky appearance, ugly patches here and there, otherwise the stain/poly came out looking really nice (I am unable to post a photo for whatever reason). Nothing appeared out of the ordinary prior to poly application. It is quite ugly and absolutely would have ruined a "real" project. I want to understand what happened.

So I ask you, those with knowledge of finishes and finishing.

Any thoughts on what I did wrong here?

and more importantly,

How can I prevent this in the future?

Many thanks from a total novice.

[tlt]

was this glossy poly or satin?

i've had trouble before with satin poly because i didn't stir it enough and the 'dirt' (for lack of a better term) that knocks down the sheen wasn't evenly distributed.

[alamode]

Semi Gloss poly, and it could be a mixing issue (although I did stir it).

[scmhogg]

Water or moisture will cause the milky spots. But, you guys don't get much rain or humidity up there, do you.

Steve

[cabinetman]

A little more information might help, such as:

Which stain was used?
How was the stain applied?
Why three coats?
Was it an oil base polyurethane?
How was the polyurethane applied?

Giving specific details provides more info to figure out what happened. Sometimes the wording in a question can be taken more than one way. For instance just saying "poly" could mean either water based or oil based. Using the words "fast dry" could apply to water based because it does, but Minwax uses that term for oil based polyurethane.
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[alamode]

Well, everything is oil based, stain and poly. I did the three coats of stain to get the shade "right", but it probably could have gone for just two coats.

I used a brush (disposable cheapo chip brushes) for everything with a fresh brush each coat.

The poly got a gentle rub down with steel wool between coats.

For the most part the finish looks very nice, and if the blotches didnt appear it would be perfect. The blotches have a similar appearance as when you leave a glass of ice water on a surface and the condensation ends up leaving a circle of discoloration on the table. In this instance they are lined up with the direction of my brush strokes in just a couple of patches (in highly visible locations).

I can see moisture being involved because it the poly was applied when it was warm, and then the garage cooled significantly overnight. There was some condensation on the surface by morning. Before the next coat was applied I wiped everything down with paper towels, and it seemed dry, but there easily could have been some moisture lurking around.

[cabinetman]

You're probably correct in your assessment. Temperature changes and material can be the culprit. It may not even be a moisture problem like being wet. Your stains may not have completely cured and were off gassing. That 24 hr suggestion about being dry doesn't mean cured.

What may benefit your finishing are some simple things to try out. Cheap brushes will give a cheap finish. You might try wiping stain on and allowing more cure time. Finishing shouldn't be rushed, as it's the last thing you need to do.

Wiping on oil base polyurethane may produce a smoother finish with less problems than brushing. Thinning the media and using lint free "T" shirt material folded up into a handy pad will lay down a thin smooth application without excessive thickness to the layer, no bubbles, and no brush marks.

In between coats, try a non woven synthetic microfiber pad, like in the Scotchbrite choices. Steel wool breaks down and can leave metallic debris like tiny shards.
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[alamode]

Thanks Cabinetman! Those are all good suggestions and exactly the kind of info I was looking for. I had that issue with the steel wool before, and so this time I tried to be extra-careful about not leaving any behind.
Live and learn, thanks again!

[Charlie]

Deleted by me.