polyurethane on table top

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Old 05-04-08, 09:42 AM
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polyurethane on table top

does anybody have a recommendation for putting on polyurethane on a large table top? i just built a really nice table and it's so big that it takes a long time to brush it all on and by the time i'm done it dries with brush marks. i've already applied three coats so i don't want to make it too thick but it just doesn't look good at all. i'm wondering if i should try rolling it on with a fine nap roller and having somebody help me do it really quickly and do one more thick coat. i'm also wondering if i should try to sand it before the last coat to make the last coat nice and flat. any ideas what i can do from this point to get a nice finish? thanks- brandon.
 
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Old 05-04-08, 10:38 AM
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First, you need to sand between each coat - it's an adhesion issue.

Second, if it's drying too fast, you could thin it with mineral spirits so it takes longer to dry. Might need an additional coat that way.
 
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Old 05-04-08, 10:55 AM
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i did give it a fine sanding between each coat with a 220 grit. i'm just not sure what will happen if i do one more coat on top of this bumpy ripply surface that i have right now without sanding it way down with an orbital or with a heavier grit sandpaper. i guess i'm not sure if one more coat will fill in the ripples or if i need to really sand them out before the last coat.
 
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Old 05-04-08, 01:50 PM
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If the finish you have applied is "roped up" you need to sand it and make it slick again before applying more poly.

Are you using a good china bristle brush? Is it sized appropriately? As noted a little thinner may help it flow better.

Using a roller can be benificial to those with poor brushing skills but it will leave a little roller stipple in the finish. You can roll it on and then tip it out with a brush to remove the stipple.
 
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Old 05-04-08, 04:46 PM
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Extend the working time.

Work cold. With any finish, low temperature = longer working time. In fact some finishes (like gel-coat) will flow for days if the temperature is low, then set up in minutes when brought to room temperature. Take your project to the basement, refrigerate the can, use your imagination.

Work under ideal humidity. Many (but not all ) polys are catalyzed by moisture in the air & wood. Reduce the catalyst, working time increases dramatically. The manufacturers do not bother to explain this, and you'll have to find it out by observation of a single product applied under different conditions. Under the porch, overcast weather, may be just impossible.


I do large areas by pouring on the polyurethane, then quickly rollering it to even thickness, then stroking over with a wide, soft brush. A table such as yours, Brandon, I would tackle by pouring about 1" wide, full length of grain and near one edge, then roller that around (not spreading too thin), then start running the brush with the grain to smooth bubbles, blips, puddles. Next pour I'd aim to start my 1st brush stroke a few inches into where I'd just been.

Unless you want an extremely high gloss finish, the final coat of poly may be wiped on very thin with lintless cloth and essentially buffed. This can't possibly leave brush marks!
 
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Old 05-04-08, 07:23 PM
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thanks for all of the tips, this forum has always been very helpful and i try to help others in return. i'm going to try rolling it on and then i'll have a friend of mine right behind me with a big brush. i have a fridge at the shop so i'll stick the poly in there for a couple hours before i make the attempt. i'm still not sure if my last coat is bad enough that it needs to be re-sanded, after what everybody has said i'm leaning towards not sanding it down much and seeing if one last heavy coat levels it all out. i guess if this last attempt doesn't work i can try to sand it all off and start over.
 
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Old 05-05-08, 02:46 AM
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I wouldn't put the poly in the fridge! Oil base coatings tend to be thicker when cold and can be more difficult to apply.
 
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Old 05-05-08, 09:02 AM
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One more thought:

When I use a polyurethane in my refinishing work I brush apply the first two coats, sand the last dead smooth with an orbital sander, wipe clean with a tack rag, and apply the final coat using an aerosole of the same brand and sheen.

I wind up with a smooth top of the sheen I want without the problem of brush marks.
 
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Old 05-05-08, 10:15 AM
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My favorite method of applying poly on a large surface is wiping it on. It takes more coats, but it's almost foolproof. You still need to sand between coats.

You can purchase wipe-on polyurethane or you can make your own by mixing regular polyurethane 50/50 with mineral spirits. It's much cheaper that way. Of course, this assumes that you are using oil based polyurethane.

Also, I agree with marksr. If the surface is "roped up" or is rough or uneven with nibs or whatever, then you should sand it smooth before continuing.

Good luck,
 
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Old 05-05-08, 08:57 PM
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i just put another coat on a few hours ago and it looks awesome this time. i think my main problem was i took too long trying to brush it all smooth instead of just getting it on and leaving it to level on it's own. doing a heavy coat filled in all of the ripply spots to where it looks almost perfect now. next time i'm trying the wipe on poly, it sounds like it would be easier and look better.
 
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Old 05-06-08, 03:51 AM
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I've never used the wipe on poly and I know there are some that like it but it would seem to me that the coating film would be too thin to give long wearing protection.
 
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Old 05-06-08, 09:35 AM
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marksr,

You're correct. The wiped on layers are thinner. It takes more coats but you can achieve the same protection in the end. The reason I like it is that you don't have to worry about runs or brush marks.

By the way, I am certainly no expert when it comes to finishes. It's the part of furniture making that I would gladly hand over to someone else if I could. :-) However, I frequently visit the forum at Wood Magazine and there a few very knowledgeable contributors in the Finishing category on the forum. Their advise on finish thickness is usually "less is better." In other words, apply the minimum thickness required to achieve the desired quality in appearance. They offer some interesting arguments to back that up. All I know is that I still have a lot to learn.
 
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Old 05-08-08, 02:22 AM
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I personally like a really thick "there's wood under all that plastic" look. So I brush just almost to the point of dripping. But I often wipe a thin coat early on just to protect the wood/stain from glue, fingerprints, paint, humidity, etc. And a thin wipe after final sand leaves a very flat surface.
 
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