Bumps in polyurethane


Hi all,
I'm trying to put a polyurethane coat onto a bookcase I'm building. Since it's a prototype, it's made out of cheap, soft pine. I've got it stained, and I'm trying to put on the final coat of polyurethane. But I keep getting small bumps about 1 mm in diameter in the polyurethane coat. If I sand them down and put on another coat, they come back (although I'm not sure if they're in the same spot).
This seems to occur no matter how thick or thin I put the polyurethane on. When I apply it, I've tried both not dragging the brush across the rim of the can and just knocking off the excess. I wipe down the surface after sanding.
I think I've got about five or six coats of varying thickness trying to get rid of these bumps. What the heck am I doing wrong?
Thanks in advance.
--
Michael White "To protect people from the effects of folly is to
fill the world with fools." -Herbert Spencer, 1891
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I will give it a try Michael, Are the bumps small craters, likely the remnants of bubbles breaking the surface and then drying as they burst? If so I would add some solvent to extend the drying time and switch to using a rag with the diluted poly if this does not work. Light coats are almost always best. Sometimes the material can start to polymerize in the can forming small crystals which you need to filter out or buy a can from a different batch. Cheers, JG
Michael White wrote:

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Are you "painting" in on Mike, or spreading it on? Poly does not like to be painted. Use a good natural bristle brush or a foam pad applicator and spread the poly in one direction, as opposed to brushing it on as you would paint. Lots of folks work the poly too much with the brush and inject air bubbles into their finish and it sounds like this might be biting you.
I understand that you want to solve this problem, but on a different note, since you have a build up of poly already, at least be aware that you do have another recourse now. Sand down your finish with 1000 grit wet paper and use a rubbing compound to bring it back to the shine you want. You have enough poly there, after 4 or 5 coats to be able to do quite a bit of sanding before risking a burn through.
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Michael,
I can think of a couple of reasons for imperfections:
* Bubbles * Dust particles (either airborn or reintroduced to the can) * Cured chunks of poly contaminating the can.
How to mitigate the problems up front:
Bubbles: Thin the poly a bit (maybe 4:1 poly to thinner ratio) and watch your application technique, as others have already suggested.
Airborn dust. Vac the area before finishing (but not right before, let the kicked up stuff settle) wipe down your work with a tack rag. Thinning will help with this as well. A thinner layer (created by the application of less viscouspoly) will create a smaller "dimple" when the dust lands on your work. Personally I thing full strength poly is only for "quick 'n thick" protection, but appearance does not matter finishing.
Contamination: Always pour into a separate container and work from that. Never pour back the extra. I find the yogurt cups work very nicely for this... they come with lids too.
Chunks... It's time to buy a new can of poly.
How to recover from the imperfections that you have (there will always be some unless you finish in an IBM silicon wafer production facility):
A cabinet scraper is your friend. This will knock off the high spots and not remove any other finnish. If you don't have one, scrape the surface lightly with a utility knife blace (just the blade, not in the knife) with the blade scquare to the surface, and moving in the direction of the face of the blade. You want to scrape, not cut. A light touch will knock off the dust nibs .
Without the high spots, your between coats sanding will only lightly abraid the surface. You should not be trying to level the finish. Use a block.
Cheers,
Steve

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Thanks for everyone's input. To address a few questions:
- The polyurethane is oil-based. - I've been careful not to paint the polyurethane on, and go over it once at the end in strokes that span the length of the piece. - They're definitely bumps, not craters. If I scrape them off, they're polyurethane chunks. - The bumps are quite numerous, and especially stand out in certain lighting. - I don't think I'm using the wrong brush, as it said it could be used for oil-based polyurethane.
Some things that were said that may have a high probability of me screwing up:
- Contaminated polyurethane. I don't remember these bumps on the early coats when the can was new, but I didn't really pay attention either. Also, I did inadvertently leave the lid off for several hours. - It might be dust. I'll clean up a little better this time and see if it makes a difference. - I may be working the polyurethane too much as stated by Mike Marlow.
Larry Jaques, you recommended a "real finish" - what were you referring to? I noticed that they have expoxy coatings, but the stuff I saw at Home Depot was $25 for a quart that covers four square feet. Rather expensive.
Thanks again!
--
Michael White "To protect people from the effects of folly is to
fill the world with fools." -Herbert Spencer, 1891
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On Fri, 18 Mar 2005 01:29:07 GMT, the inscrutable Michael White

ABP. (_anything_ but poly)
Ackshully, poly isn't bad on its own, especially the oil-based stuff. It's just that about, oh, 95% of people using it slop it on and it looks as if the piece was wrapped in Saran.
Either toss that can out and buy some Waterlox (my current favorite, and a gallon of it arrived at my door last Saturday) or get a new can of that crap (or shellac or Watco) and be sure to filter it with a paint-store sieve this time. Paint filters are inexpensive and worth their weight in gold for a nice finish, especially with old cans of pukey poly.
You didn't mention trying a magnifying glass. Did you make sure it wasn't bubbles (popped or unpopped) you were seeing?
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On Fri, 18 Mar 2005 01:29:07 GMT, Michael White

(good answers snipped)

In one of George Grotz's books, he states that for ANY varnish product you need:
-A scrupulously cleaned room.
-A NEW can of varnish
-A NEW brush.
Every time I've done all three, I've gotten good results with anything in the varnish family.
When I haven't, I've done a fair imitation of Jurassic Park what with all the insects trapped in resin and all.
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As for the brush questuion, I solved the same problem by buying a $15 spray can from Harbor Frieght. My other prob. has been temperature. In the winter I had a real tough time getting a good finish. Not being able to heat my shop suffeciently, what I have to do now is put the project, and the can, in the house overnight if spyaying in the moring; problem solved.
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On Thu, 17 Mar 2005 04:39:20 GMT, Michael White

I think JGS got it right. You might be going cheap all around. It's likely lumps of polymerised urethane. Try sanding down with very fine wet paper. Use a block, and don't press too hard, but lots of sideways motion, not pressing. That is, take your time. Try on some scrap material first after varnishing and drying adequately.
If so, go buy some new varnish and use the old to start the next outdoor scrub-brush cleanup.
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If only a few small bumps, filter the poly before using it. Otherwise, toss it and get a new can. I'd remove what's there, and not try to sand it down. Especially not with the 1000 grit mentioned, unless you have alot of time. You could wet sand with 320 then 400 and recoat, but I suspect you'll still have embedded pieces of cured poly that'll be visible.
Others suggested thinning, but that will rarely help. They also didn't ask if you were using water or oil base poly, as the former should not be thinned for hand application. As to bubbles versus bumps, if a real thin coat has them, they're not bubbles. As to adding solvent to slow it down, it depends on the solvent, as some will speed it up. And if it's water base and you add solvent, it'll be a real mess.
GerryG
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On Thu, 17 Mar 2005 04:39:20 GMT, Michael White

Get a magnifying glass. I'm sure you'll find that they're bubbles introduced by either the wrong brush or the wrong use of a good brush. Why not try a REAL finish instead of poly next time?
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Real? What's unreal about poly? I'm not a fan of applying slow drying solvent based poly, but to suggest that poly's aren't real is quite a stretch of the imagination.
Dave
Larry Jaques wrote:

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Poly, urethane, acrylic are all suitable finishes for certain applications when applied correctly. Lacquer is an interesting one that many endorse, but the name itself is used for many different and sometimes unrelated compounds. Except for shellac, many of them are plastic of one type or another. And even shellac can be applied so that the result looks like a cheap plastic coating. But, it does give us something rather harmless to argue about.
In the meantime, I'm a'goin to go and stain some cherry. GerryG

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On Sat, 19 Mar 2005 02:44:03 GMT, the inscrutable GerryG

Ah! The key words are those last 3, which rules out most applications in the real world. The way poly is applied in the real world is akin to painting a Lambo with a latex and a pig bristle whitewash brush.

<g>
...and come back as a crotch crab in your next life.
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On Sat, 19 Mar 2005 06:47:28 -0800, Larry Jaques

their "cherry". Had to go to Jewitt to find out their finishing schedule, but at least I managed to match it without using the glaze. I will say that everybody liked the cherry better before it was stained, but the budget didn't allow making a new coffee table also. GerryG
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On Sat, 19 Mar 2005 15:02:56 GMT, the inscrutable GerryG

Thank you. It's not often that someone who actually uses the crap will agree about its darker half (95%).

Did not. You could have said "Get some jerk kid to ruin it for you." Alternatively (if you wanted your daughter to speak to you again) you could have stated it a bit more diplomatically "Let me finish it and see what you think. If you don't like it, we can make it look shitty like your commercial piece of crap there with some nice Minwhacked PolyShades." (Grain? What grain?)

There ya go with the YB talk...

That's good.

Hindsight's 20/20, eh? <sigh>
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On Sat, 19 Mar 2005 18:20:49 -0800, Larry Jaques

mix of TransTint dyes, using several feathered coats to blend the cherry pieces to the same shade. Sprayed with clear shellac to seal and scuff sanded, then a light coat of amber shellac to darken it just a bit. Finish was three thin coats of flat poly. With no pigments or glaze used, the grain is still there. GerryG
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I assume they're tiny bubbles or dust specks. I've used polyurethane quite a bit (I use a foam brush)and always have tiny imperfections in the final coat. After it cures for several days I sand the final coat with 600 paper by just barely touching the surface with the sandpaper. This super light sanding does not change the sheen and removes the tiny imperfections (at least to the touch). If I look very closely in the right light I can still see some very tiny imperfections, but the surface now feels smooth.
Charles
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