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Thread: Polyurethane looks horrible

  1. #1

    Polyurethane looks horrible

    I put the first coat of polyurethane on some oak plywood shelves last night, and it turned out horrible. The surface has a bunch of little tiny rough bumps on it.

    The polyurethane looked bumpy when I first put it on, but I guess I figured that it would automagically get smooth as it dried. Needless to say, I was wrong.

    I'm wondering if it might be because the polyurethane I was using is so old.

    I did try putting on a really thin coat on a couple of the shelves, and they turned out a lot better. Maybe I should just do that.

    Any other ideas?

    And once I figure that out, what should I do with my bumpy shelves? I'm planning on taking a sanding block and lightly sanding the bumps down. Is that a good idea?

  2. #2
    Join Date
    Dec 2006
    RETIRED(!) in Austintown, Ohio
    Oil or water based poly?

    Water based will definitely raise the grain (a lot!) on oak. So will the first coat of oil based, but not as badly as the water based stuff.

    If it's already dried overnight, just sand the bumps smooth - 220 grit works well for this - and recoat. If you didn't use a filler, it'll probably take several coats to fill the pores in the oak's grain.

    Personally, I like to first use pore filler, then seal everything with a coat of shellac before putting the poly on it. Works a lot better for me that way. YMMV...
    Jim D.
    Adapt, Improvise, Overcome!

  3. #3

    So, this wood filler -- is that what they call a sanding sealer?

  4. #4
    Join Date
    Nov 2006
    This may be a bit presumptuous, but did you lightly sand or steel wool after the poly dried then apply another coat sanding again and then a third coat?
    Poly on the first two coats does feel bumpy but the more coats with proper treatment between coats will be very smooth.

  5. #5
    Join Date
    Feb 2008
    Cedar Park, TX
    Oil based poly, especially the typical brands at the big box stores, is way too thick to be applied by brush by most of us mere mortals. And stretching the finish over a larger area in order to obtain a thin coat exacerbates the problem even more. It is just too thick to level on its own if you over work it even slightly.

    Even though the can will likely tell you not to thin it, thinning with mineral spirits or naphtha even 10% by volume will vastly improve its ability to level on its own. Naphtha flashes off faster giving the film less time to collect dust nibs, one cause of the bumps you talk about.

    While you can buy all sorts of products out there like stain preconditioner and sanding sealer, a thinned coat of poly (or what ever final finish you will be using) will serve basically the same purpose. Sanding sealer is basically polyurethane with some stearates added that help prevent clogging of sandpaper in subsequent sandings.

    Grain filler mentioned previously is not that putty stuff at the big box but is generally a thick liquid available in different shades which is applied to the wood and worked into the grain with the excess being squeegied (is that a word?) off. Once it has dried, light sanding removes the remainder of the excess from the surface. A couple applications may be required if you have really deep grained wood. You can also fill the grain by using some poly as a lubricant to sand the surface creating a slurry which can then be worked into the grain and allowed to dry in the same manner as the grain filler. You can also fill the grain by applying numerous coats of your film finish, then sand back to the wood between coats.

    Another option for a sealer for the wood, particularly useful if you are staining, is a spit coat of dewaxed shellac. I like using shellac on oak and other woods susceptible to grain raising, to intentionally raise the grain, then sand using a sanding block and 320 or 400 grit paper. This is particularly good with hardwood plywood where you have concern over sanding through the veneer. That is the only sanding I generally do on hardwood ply.

    Concerning the use of grain filler and/or stain you can obtain different looks quite simply by the color of grain filler used and the order of application. For example you can stain the wood, then apply a grain filler similar to the stain color for one affect, of contrasting color for another. You can use just grain filler of a contrasting or similar color to achieve other affects. Or you can apply the grain filler then stain, again obtaining different affects by the color of the filler in comparison to the stain. You can also condition the wood by applying a spit coat of shellac at some point in there to obtain even other affects. The dewaxed shellac also makes a great barrier between your stain/grain filler and the poly or other filming finish.

    Get you some scraps, and some of the stuff mentioned and play around with it. And thin your poly so it flows better and self levels and have fun.

    "If politics wasn't built on careful deception it wouldn't need its own word and techniques. It would just be called honesty, education, and leadership."
    Bob "Phydeaux" Stewart one day on Woodnet

  6. #6
    Join Date
    Apr 2007
    No, not all of SoCal is Los Angeles!
    Quote Originally Posted by Porter Bassett View Post

    So, this wood filler -- is that what they call a sanding sealer?
    Two different things. Pore filler comes in a few different flavors:

    etc., etc.

    For a sanding sealer, I use Zinsser's Seal Coat which is a clear dewaxed shellac and compatible with almost anything. So, if you'r after a glass smooth surface, use your pore filler, sealer and then finish. If you like the more natural oak look, skip the filler.
    Any sufficiently advanced technology is indistinguishable from magic.
    - Arthur C. Clarke

  7. #7
    Join Date
    Nov 2006
    Tokiwadai, Japan
    Great info, Jerry... Thanks

    Porter, post pics if you can. I'm sure the members will get you through.

  8. #8
    Join Date
    Feb 2008
    Cedar Park, TX
    Best thing someone can do to "get" finishing is to experiment. Try stuff out on scrap and keep track of what you did to what. Long as you don't start messing with ammonia, bleach, or drain cleaners, you ought to be alright. That and properly dispose of your oily rags and such and work with plenty of ventilation.

    Finishing has always been one of my my least favorite parts of woodworking, but in the end it either makes or breaks the project.

    "If politics wasn't built on careful deception it wouldn't need its own word and techniques. It would just be called honesty, education, and leadership."
    Bob "Phydeaux" Stewart one day on Woodnet

  9. #9
    Join Date
    May 2007
    Kansas City, Missouri
    One suggestion that I got from the wood show recently was to use a sample board. The board(s) should be from some of the same ones used on your project. Use these to test your finishes prior to finishing your project. If it doesn't work well, try something else on another sample board and you don't have to sand down the project and start over.

    It was mentioned at the show that if your getting bubbles, thin down your finish with whatever is recommended for cleanup or a recommened thinner. Sometimes your brush can be the culprit as well, so make sure your using a good brush.

    To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault

  10. #10
    Join Date
    Mar 2007
    Humm thinning? ...think I will try this next time.

    This is the general procedure when I apply Polyurethane with a brush.
    I generally use fresh can of polyurethane for each project if any amount of time between projects, say 6 months or more.

    Sand to obtain a smooth uniform surface. Remove all dust with a cloth dampened with mineral spirits and then a tack cloth.
    Lightly Stir Polyurethane before and during use to eliminate settling on the bottom of the can. Stir in such a manner as to rotate the product from the bottom to the top of the can. NEVER SHAKE.
    Apply a THIN coat Polyurethane using a high-quality natural brush.
    Let dry 6- 8 hours. Then lightly sand entire surface with fine sand-paper (220 grit) Remove all dust with a cloth dampened with mineral spirits and then a tack cloth. Apply second coat. Let dry 6-8 hours. and for a real smooth surface...
    Rub the surface with Extra fine 000 steel wool, Remove all steel wool dust with a tack cloth. Apply the final third coat
    After final coat, allow 24 hours before light use.
    Last edited by Dave Trask; 02-13-2008 at 03:08 AM. Reason: can't spell worth a dam!
    Don’t have the best tools, don't use the best woods and projects don’t always turnout perfeck.
    I just feel the need to work with wood!

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