HELP !!! how to get a glass smooth finish with polyurethane

[Jim Colombo]

I have a desk top that is 5' x 30" and I'm finishing with water based polyurethane. I used a brush on the cabinets and I have tiny bubbles in places that are ok on the cabinets but would not be on the top. How can I avoid the bubbles, I'm being as careful as possible when brushing but still end up with some bubbles. Even sanding still leave little bumps.

I don't have a spray gun so I need to go with a brush (which are top notch). What can I do to minimize or better still eliminate the bubbles.

Do I need a material other than polyurethane?

Thanks,
Jim

[Julie Moriarty]

After you lay down the protective coats, sand smooth and follow up with wipe on poly. Keep laying down the coats until you're happy with the finish.

[Jim Colombo]

Thanks Julie. I love your diaper joke, so true.

[Steve Schoene]

In general bubbles in the finish means you need to thin it. With waterborne that is tricky since you can add only a very small amount of water--well under 10%. You can thin a bit more with extender made specifically for waterborne finish.

If what you see isn't really bubbles but is included dust, then the solution is different. I've never brushed on any finish that hasn't collected a little dust. If the amount is really small, the first thing to try, is to rub it out with crumbled up grocery bag paper. It is a really fine abrasive that may remove the dust nibs without dulling the overall sheen very much.

If it will take more effort you would have to rub out the finish. To do that first let it cure for several weeks, since the harder the finish the better. Then use the finest sandpaper, used by hand over a flexible block, that will remove the blemishes. (Usually this is is no coarser than 600 grit.) Then go to a finer grit paper to achieve an even sheen--something like 1200 and then 2000 grit. If this isn't shiny enough use polishing compound. (Not rubbing compound.)

[Wayne Lomman]

In the absence of a spray gun, hand apply as Julie says. It's the best way to get a smooth finish.

As Steve says if the problem is really dust, you need to address getting rid of it. If it is bubbles they can be caused by mixing too vigorously in the can as well as by the brushing action. Check that as well. Finally, how is the humidity? If it's too dry it upsets the curing mechanism. 60% is generally the minimum. Cheers

[Jason Lester]

What finish are you using? Some level better than others. General High Performance levels really well with a foam brush for example. I've never seen bubbles in it.

[John TenEyck]

If you add 3 to 6% of GF's Extender to any WB varnish it will slow down the drying rate and let the bubbles burst instead of drying in the film.

John

[Doug Hepler]

Jim,

All good suggestions so far. I agree with the recommendation that you use wipe-on varnish as suggested by Jule and Wayne.

I use oil based varnish applied with a brush. After I have my base (2-3 coats) I often thin the last coat or two up to 50% so that it will flatten and the bubbles can pop as John suggested.

One suggestion that has not been offered yet is to avoid wiping the brush on the side of the can after dipping it in the finish. This usually introduces bubbles (you can often see them accumulate on the surface of the liquid in the can). Dip the brush very shallowly and let the excess drip off if necessary. If the brush is a bit overloaded this will resolve when you brush it out. Finally, if you are using a high quality brush, tip off each strip before you go on to the next. I admit that none of this will be necessary if you use wipe-on varnish, but I had to offer my advice from the olden days.

Finally, you could rub out the final coat. Since you are using water based varnish you should not use steel wool, but the 0000 synthetic equivalent, will give you what you are looking for. I would not rub out fewer than 4 coats, but 0000 synthetic steel wool is very mild unless you get carried away. Follow the rub with paste wax in a week or two after the varnish has cured.

Doug

[Jim Colombo]

In the absence of a spray gun, hand apply as Julie says. It's the best way to get a smooth finish.

As Steve says if the problem is really dust, you need to address getting rid of it. If it is bubbles they can be caused by mixing too vigorously in the can as well as by the brushing action. Check that as well. Finally, how is the humidity? If it's too dry it upsets the curing mechanism. 60% is generally the minimum. Cheers

I'm in Arizona and the humidity was less than 25%.

[Jim Colombo]

What finish are you using? Some level better than others. General High Performance levels really well with a foam brush for example. I've never seen bubbles in it.

I'm using MinWax Polycrylic, but I'm not married to it. I can change if everyone feels it isn't the best.

Jim

[Jim Colombo]

In general bubbles in the finish means you need to thin it. With waterborne that is tricky since you can add only a very small amount of water--well under 10%. You can thin a bit more with extender made specifically for waterborne finish.

If what you see isn't really bubbles but is included dust, then the solution is different. I've never brushed on any finish that hasn't collected a little dust. If the amount is really small, the first thing to try, is to rub it out with crumbled up grocery bag paper. It is a really fine abrasive that may remove the dust nibs without dulling the overall sheen very much.

If it will take more effort you would have to rub out the finish. To do that first let it cure for several weeks, since the harder the finish the better. Then use the finest sandpaper, used by hand over a flexible block, that will remove the blemishes. (Usually this is is no coarser than 600 grit.) Then go to a finer grit paper to achieve an even sheen--something like 1200 and then 2000 grit. If this isn't shiny enough use polishing compound. (Not rubbing compound.)

I'm pretty sure they're bubbles. I get tiny circles after sanding.

[Wayne Lomman]

25% is too low for good film formation. Either wait for better conditions or hose down the floor before you start work. Putting a tub of water in front of a fan helps too. Read the tech data sheet for your product and it will most likely say 50-60% humidity is best. Cheers

[roger wiegand]

I wait until the poly is fully dried/cured (at least two weeks) then rub it out as you might with any other varnish-- fine (600+ grit) sandpaper if necessary, 0000 steel wool followed by polishing compounds to give the desired sheen level. I've never actually tried for a very high gloss, so I don't know how that would work, but it works very well to achieve the semi-gloss sheen I'm looking for and a perfectly smooth and very nice feeling surface.

I've also never tried it with a waterborne polyurethane, so can't speak to that. I don't use the stuff.

[Paul Follett]

I use gallons of water based Poly mostly Varathane Ultimate with no problem with bubbles. I use a good synthetic brush and never, and this is key shake. I stir slowly to avoid having billions of bubbles that will never seem to go away. I also live in a area where summer humidity is in the 20's or lower as long as I am in the shade I can get a good finish.

Sand
Stain
1st coat Poly
Sand with 320, lightly
Recoat
Repeat sand and Poly usually 3 coats for me.

[Jim Becker]

Water borne finishes...including the Polycrylic...will perform much better if sprayed. They dry so quickly that it can be a challenge for them to lie down, resulting in the bubbles, etc. For brushing, the actual brush is important, too...nothing serves you better than a very high quality brush if that is your preference for application. Oil based varnish is better, IMHO, for brushing or wiping because it dries slowly and can be more forgiving as it levels out.

[Jim Colombo]

Water borne finishes...including the Polycrylic...will perform much better if sprayed. They dry so quickly that it can be a challenge for them to lie down, resulting in the bubbles, etc. For brushing, the actual brush is important, too...nothing serves you better than a very high quality brush if that is your preference for application. Oil based varnish is better, IMHO, for brushing or wiping because it dries slowly and can be more forgiving as it levels out.

I already have 2 coats of the water based poly on the tops. Could I still go to the oil based poly and put it on top of the coats already there?

Jim

[Jim Becker]

I already have 2 coats of the water based poly on the tops. Could I still go to the oil based poly and put it on top of the coats already there?

Jim

Shouldn't be an issue as long as you lightly abrade the finish already applied. It's best if it's had some time to cure a little, too. Water borne "poly" isn't anything like the oil based product. It's an acrylic that has some level of polyurethane resin in it. It also doesn't have the "warm" look of the oil based product. The light abrasion is to insure that the other finish will adhere to it....polyurethane finishes are not fond of sticking to themselves sometimes. :)

[Curt Harms]

Shouldn't be an issue as long as you lightly abrade the finish already applied. It's best if it's had some time to cure a little, too. Water borne "poly" isn't anything like the oil based product. It's an acrylic that has some level of polyurethane resin in it. It also doesn't have the "warm" look of the oil based product. The light abrasion is to insure that the other finish will adhere to it....polyurethane finishes are not fond of sticking to themselves sometimes. :)

Would shellac be appropriate as a barrier coat between water based and oil based? I've never tried shellac except spraying but it dries fast. I find that dewaxed shellac on raw wood under waterbornes works very well, there's no grain raising and amber or tinted shellac can add some color if desired. A light sanding and succeeding coats are smooth as a baby's butt. A little late for Jim but something to think about going forward.

[Jim Colombo]

Thanks for all the input but before I was able to implement anything, disaster struck. The panels (composed of laminated strips of poplar) both curled up on the sides (looks like boat bodies). I'm guessing it has something to do with the moisture in the 2 coats of poly already applied.

They curled up on the sides with the poly. Now I have to figure out if I can save them.

Any help in that area would be greatly appreciated.

Jim

[Bennett Ostroff]

If you haven't finished the bottom, you'll need to. That's likely the culprit of the cupping - uneven moisture absorption. Flip it over, put 2-3 coats on the bottom, and I suspect it'll be back to how it was in a day or two. Also, the thinner the panel, the more susceptible to cupping.

[Jim Becker]

Would shellac be appropriate as a barrier coat between water based and oil based? I've never tried shellac except spraying but it dries fast. I find that dewaxed shellac on raw wood under waterbornes works very well, there's no grain raising and amber or tinted shellac can add some color if desired. A light sanding and succeeding coats are smooth as a baby's butt. A little late for Jim but something to think about going forward.
Yes, De-waxed Shellac is often used as a barrier coat. I generally mention that, but I honestly just plain forgot. Thanks for the reminder!