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Thread: wet sanding poly for a smooth finish?

  1. #1

    wet sanding poly for a smooth finish?

    I've learned a ton from this site and have a question that I can't find an answer for.

    I am applying Minwax fast drying clear gloss poly to a piece of Birch plywood sanded to 220.

    I used a new foam brush for all coats and all coats were 100% (no thinning)
    1st coat was real thin at 100% Dried 10 hrs. Sanded lightly with 220, 2nd coat was also thin, dried 14 hrs and sanded at 220. I noticed slight shiny valleys and dull ridges which I would guess means that the brush strokes are showing up. I sanded with 220 again and applied another thin coat this morning.

    Is there a way to remove the small ridges to get a smooth finish? I seem to remember hearing that you can wet sand to get them out but have I gone too far already to make that successful? I do plan on rubbing it after a few weeks of drying. Will this take the ridges out?

    Thanks in advance

    Brett

  2. #2
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    It sounds like you may have added additional film thickness (re-coats) before getting your surface as flat as you'd like. The building process goes in stages and you are building with the same finish so at worst, your in for some sanding and probably another coat or so.

    Each coat gets sanded and this should flatten off the "tops" of the hills and reduce the "depths" of the valleys. The end goal of adding another coat and sanding between is, of course, to get as close to "flat" as you can. If your surface is flat work, use a sanding block to keep things moving toward 'flat'.

    Now that I've probably told you nothing you don't already know; Jeff Jewitt spells it out pretty well here and describes the steps leading up to and including rubbing out. Good luck and I look forward to seeing your progress.
    Last edited by glenn bradley; 02-07-2008 at 04:54 PM.
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  3. #3
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    thank goodness for jeff!!!

    huh, he is a wealth of info in finishing and very willing to share his expiernce.
    If in Doubt, Build it Stout!
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  4. #4
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    wet sanding poly for a smooth finish?

    I am not registered on the Fine Woodworking site so I didn't read Jeff's input, but I agree with Glenn.

    In trying to get a surface finish as flat as you can, (my personal experience comes from custom car painting, where the glass is sometimes not as flat as the paint) you must use a sanding block. And, it is not just for the flat surfaces. A flexible block or pad will do wonders in knocking down the highs and bringing up the lows as you apply finish coats and sand them down even on curved surfaces. Anything is better than your fingers, because you need to bridge the low spots while cutting off the high spots.

    After making sure your finish coat has dried enough, because it will continue to shrink down even after you sand it flat as it dries, sanding with an appropriate block or pad will get you closer to the disired flat surface you are looking for.

    Yes, you can wet sand, but unlike car finishes, you don't want water soaking into your wood so don't wet sand until you know you have enough coats applied to ensure a good seal. Also, when you wet sand, dont cut too deeply so as to cut through and expose wood again. It is better to sand lighter and apply more coats that to try sanding flat in one sanding.

    After you have your final smooth coat and it has dried completely, you can rub the finish to a high gloss rubbed finish, which will differ from the wet look of a fresh unrubbed finish coat.

    Hope this helped some.

    Aloha, Tony
    "You got to learn from the mistakes of others. You won't live long enough to make them all yourself". (Author unknown)

    "Time flies like..... an arrow,,,Fruit flies like..... a banana." Groucho Marx

    Ah,,,to live in Paradise!

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  5. #5
    Thanks for the replies guys. I think what happened was that I failed to notice that after the first coat, I did not sand enough to get the brush strokes out. I was using a sanding bock with 220. I was afraid to sand through the first thin coat. When I applied the 2nd coat, I started to notice that after sanding, I would have small peaks and valleys which I think are from the brush strokes. ( dulling the high spots and the valleys would remain shiny)

    This time, I took some 00000 steel wool and put some elbow grease into it and then hit it with a fresh piece of 220 paper on a block and I got rid of 85% of the issue. I applied another coat of poly and things are looking better. After a couple more coats and a good cure time, I think I can rub it into a decent finish.

    by the way, this will be a wood racetrack for a poker table that I am building.

    Thanks again!
    Brett

  6. #6
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    Sometime if you put on a coat that is to thin, it can not level out real good. If you go to heavy it will take to long to dry. One thing that might help is trying a different stlye brush. I bought one made for use with poly and it left a better finish then a foam brush. I do not think it is possible to get a poly finish that is pefectly level. There will always be some leveling required.

  7. #7
    Thanks for the tips. I ended up buying a brush made for poly as suggested and used it during the last 2 coats. Between coats 3-4-5 I rubbed pretty aggressively with 0000 steel wool and followed it with a light sanding of 220. I also went just a bit heavier on the coats to allow it to even out some.
    There are some areas that I'm not real happy with but overall it came out well.

  8. #8
    Spit Can the foam brush as it will leave marks no matter what (my opinion) I prefer a soft cotton Tee-shirt cloth to wipe on poly (not just Wipe-on Poly but full strength Poly)

    As for fear of wiping out the first coat when you sand, That usually happens, For the most part, the first coat is sacrificed to use as a filler of low spots and pores, etc. Removal of the first coat would eliminate those marks and ridges. Most fellows overwork the poly, Back and forth strokes are a killer, also when Poly settles out it will float the strokes away but in doing so it goes through some surface changes and as we look back over our work we see the light reflecting differently and think "Oh, I messed up there and didn't apply right, I thought I did" and of course the tendency is to re-stroke that spot. But in fact you created a restart of the floating out process and if you disturb it too much it begins to set up in a wavy state. That setup process needs to be left alone. Going back to get that little bubble often creates more work, later. Apply a simple stroke smooth layer and "LEAVE IT ALONE" is the best practice, I know it is hard to keep your hands out of the process but just KISS (keep it simple, etc.) You will find it will resolve itself and be surprised how well it looks.

  9. #9
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    wet sanding poly for a smooth finish?

    Nice looking job Brett,

    But, IMHO, I think you are going about your sanding backwards from what I was taught. It has always been "sand, with a block first, to level, then rub out with steel wool". Unless I learned it wrong, but it has worked real well for me all these years.

    Try it,,,you'll like it!

    Aloha, Tony
    "You got to learn from the mistakes of others. You won't live long enough to make them all yourself". (Author unknown)

    "Time flies like..... an arrow,,,Fruit flies like..... a banana." Groucho Marx

    Ah,,,to live in Paradise!

    Registered voting member

    Fighting for all I am worth, and praying every day.

  10. #10
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    Wow, that's quite a poker table.
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