We finished an old (I suspect original) oak floor in the master bedroom of our 1910 house. It had carpeting and when we lifted a corner we found the hardwood floor. Turns out SOMEONE at some time in the past decided it would be a good idea to texture the ceiling and that they didn't need drop cloths. So when we exposed the floor it had white paint, texture, dirt, some kind of other smutch and all sorts of yuck. In addition it had been sanded at some point, and PARTS of the floor had a coat of varnish, while other areas didn't.

We got all the stuff off the floor, cleaned it a couple times with denatured alcohol, scuffed it with 100 grit sandpaper and put our first coat of Minwax semi-gloss polyurethane. It looked UNBELIEVABLY good. We thought it would be so-so, but the varnish blended and covered and the floor looked amazingly good. So, NO problems with the wood, the floor, the old whatever, or anything else.

Scuffed the whole thing with 220 on a pole sander, cleaned with denatured alcohol, and put the second coat on...

And NO, we did NOT shake the can of varnish......

Looked even better. Any imperfections from the first coat blended, covered, and the floor looked fantastic. No runs, no drips, no bubbles, no problems.

Let it dry again for 12 hours, scuffed it with the 220, cleaned with the alcohol.
Our first 2 coats finished off the original gallon so we opened the second gallon for the third coat. And STIRRED it well. NO SHAKING...

Applied the final coat... AND GOT BUBBLES. Tiny, little bubbles that you can see if you look just right. And once you SEE 'EM, you can't STOP seeing them.

The only thing I can figure is the second gallon was slightly thicker or somehow different, even though it was exactly the same, bought at the same time, used within a reasonable amount of time... Followed all the instructions as far as drying, scuffing, and so on.......

Bubbles. I can take a random orbit sander and use 80/100/150 paper to sand it down a little, and put on another coat, but what's to keep the bubbles from coming back?

So, without doing anything DRASTIC, what do I do? Was the varnish too thick and didn't let whatever bubbles there were come out? If so, I can thin it, but how much and with what? Do I just scuff with 220 and put another coat on? Sand with 80/100/150 (or whatever I should use) and put another coat on?

Help... I see bubbles! (which I may be able to ignore, but the domestic associate has commented on them, and she NEVER complains, so it's a problem).

 

The only 2 reasons for bubbles in the coating is a contaminant on the substrate [not likely] or air introduced into the coating by either shaking [or vigorous stirring] or improper application.

I assume you let the denatured alcohol evaporate prior to applying the poly. How was the poly applied? brush, roller or lambswool applicator?

The fix would be to sand it smooth [may take more than a scuff sand to eliminate the bubbles] remove dust and apply another coat. If need be you can thin the poly up to 10%

 

We gave the alcohol plenty of time to evaporate, and it dries really fast, so I don't think that was the problem.

I can't imagine anything different being in the air for the third coat, so I'm not sure airborne contamination is likely.....

Which leaves the possibility of improper application... We applied all three coats with special rollers made for varnish, and they worked perfectly on the first two coats. There may have BEEN bubbles from the first two, but they went away before the varnish dried. I don't recall SEEING any, but in either case, they smoothed out and the first two coats were very nice and smooth...

Or some unknown difference in the varnish - thicker, different, something, between the two gallons......

If I do sand, what grit do I sand with? And do I have to go down BELOW the bubbles or will a new top coat "fill" them and make them invisible? 'Cause I definitely don't want to try to take off all three coats and redo it...

And if I thin the varnish 10%, what do I thin it with? Mineral spirits? Denatured alcohol? Lacquer thinner?

 

You'd thin the poly with mineral spirits [assuming it's oil base, read the label]

Rollers aren't great for applying poly and might be part of the problem. When the wood is 'thristy' it will suck up some of the 'defects' in the applied finish but as the wood gets sealed it's harder for the wood to absorb them. A lambswool pad is best/quickest for applying floor poly but a brush also does a good job. I doubt you'll ever find a pro that uses a roller.

It's best to sand the bubbles down flat. While just sanding off the tops of the bubbles might work, the next coat might not totally eliminate the defect. Thinner coatings generally lay down better than heavier ones although a lot depends on how quickly it dries.

 

Just to clarify, you used POLYURETHANE to finish this floor. VARNISH is an entirely different product.

 

I suspect the OP is like me and sometimes interchanges the two, sometimes I even say I'm painting when it's actually a stain or poly/varnish

Post #1 said MinWax semi-gloss polyurethane, it would help to know if it's the oil base or water based.

 

Yes, it's oil-based polyurethane.... So, depending on what the domestic associate decides, I'll do some sanding, thin the poly, and add one more coat.......

I know the lambswool is widely used, but I've had the worst imaginable results using one. Which is why we switched to a roller specifically for applying polyurethane. And had I left it at two coats - long after the wood was sealed, we'd have been fine... I shouldn't have tempted fate, but I figured a third coat would help with durability and looks... Bad choice...

But, if necessary I'll sand it. Is there a preferable grit for doing this sanding? I want to help the bubbles with as little collateral damage as possible... Am I safe with 100 grit or should I use 150? Or will it matter?

 

Hard to say without being there to see/feel the bubbles but 100-150 grit - somewhere in there should be good. I'm thinking the roller is probably the culprit and would brush the next coat. I think they still sell 5" or 6" natural bristle brushes.

 

220 is the normal between coats grit and I would try to stay as close to that as possible. In other words, I'd make sure the 150 wasn't aggressive enough before trying the 100 and so on.

 

Yes, 220 is the norm.... Personally, 180 is the most aggressive grit I would use between coats. You don't want to sand through to bare wood accidentally, and that's what could happen if you used 100, 120, 150. Start with the finest grit and if it takes too much sanding go to the next courser grit. but my god, definitely don't use 100... and don't start with 150.

 

I'll find some 180 grit velcro pads and give it a try on the random orbit sander... I"m hoping it just takes a little sanding to clean things up.

I'll also get a wide brush for putting the last (HOPEFULLY) coat on this thing...

 

FIXED!

Put my big-boy pants on yesterday and went forth to do battle with the bubbles. Stuck some 220 on the random orbit sander and did some sanding/scuffing. I did it by feel - feel the bubbles, sand, feel smoother, sand a bit more 'til they felt smooth. Within a couple minutes I got the hang of it and got the whole floor sanded. BTW: The 220 was enough, didn't need the 180. Poly does quite a number on sanding pads! Didn't get gunked up, they just got DULL!

Vacuumed to remove dust, then washed with denatured alcohol, then let the room sit with the door shut for several hours so any airborne dust could settle out.

Did a second cleaning with alcohol and microfiber cloths to make sure things were ready.

Thinned the varnish about 8% using mineral spirits and applied it with a natural bristle brush. It was instantly apparent that things looked a LOT better. I made sure I went across the narrow dimension (it's a 15 x 20 room) and with the grain (same as I'd done with the roller) so I always had a good, wet edge as I brushed. It was slower than rolling, but the whole room was done in about and hour and a half. We both (spouse and I) checked repeatedly, up close and personal with a bright light to make sure the bubbles were completely invisible, and they are.

Floor is done, dried, and we checked this morning with the sun coming in the windows, reflecting off the floor, and it still looks really good.

Tomorrow the shoe moldings go on and we'll move the furniture back in after a couple more days of drying/curing...

Thanks for the help! Y'all helped us fix a problem of the - "If you can't see it, great, but if you ever DO see the problem you'll NEVER be able to unsee it!" variety!

 

Thanks for the update
Don't forget it takes about 72 hrs for the poly to get hard.

 

No problem. We won't even walk on it 'til late Friday

Here's what we started with:



All that white stuff is paint and ceiling texture... Somebody, sometime decided it would be a good idea to texture the ceiling and didn't need a drop cloth...

Here's what we have now

 

That looks really nice and my home has the same floors. As Mark said the problem was rolling.

The way I look at it is when you use lambs wool or a brush, you are not brushing the finish on.
The paint or varnish is flowing off the tip of the brush and will self level if you can apply just the right amount and don't go back over it.

 

I can't disagree with the assertion about the roller. But, why do they make "varnish" rollers, specifically for putting on polyurethane if they're going to cause bubbles? And why didn't we get ANY (and we looked really carefully) bubbles in the first two coats? It wasn't until we switched to the second gallon, and then we got TONS of bubbles...

Wouldn't I have gotten bubbles in the second coat? I can see not getting them with the first coat on the bare(ish) floor, but if the roller was causing the bubbles wouldn't I have gotten them with the second coat?

In either case, if I ever have to do this again I'm just going to have to LEARN to use the lambswool applicator, or just spend the extra time to brush it on. Brushing was easy, and I didn't see the problems I saw with the lambswool, but it does take some longer to do than rolling or using the lambswool...

 

They sell roller covers for varnish/poly because diyers insist on it. For many it's easier to roll a coating on than it is to brush. I suppose a roller stipple/orange peel is preferable to missed spots [holidays] Up until 30 yrs or so ago all the pros brushed on the floor finish, usually with an 8" brush.

Generally the better a substrate is sealed the more the roller stipple will show. It's possible the conditions [temp, humidity and poly thickness] of the 2nc can were more prone to accentuate the problem.