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Polyurethane is confusing me. ;)

I'm finishing a table top with polyurethane. I've sanded down the original finish to bare wood down to 220 grit paper.

I vacuumed and wiped it all down, then did a final wipe with a rag and some mineral spirits. After drying, I then put on the first coat.

It's been about 2 hours, and this is where I'm stumped.

The can says to re-apply coats after 2 hours, but doesn't say to sand. It does say that if you wait 24 hours, then you should sand.

Researching on the internet, the common advice appears to be sand between every coat.

So, the questions:

  • should one sand between every single coat?
  • how long should one wait before sanding?

My understanding of the can label is that multiple coats are fine after 2 hours WITHOUT sanding. They're also fine after 24 hours WITH sanding. Is that correct? Any thoughts on which method is better?

FWIW, this is a satin finish, as I'm purposefully trying to knock down any sheen to give it the barest-of-wood look I can, so super-smooth glossy isn't a priority on this project.

UPDATE: BONUS QUESTION!

What's everyone's opinion on 'with' vs. 'cross' grain bush application? Always go with the grain? Alternate between coats?

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6 Answers 6

up vote 9 down vote accepted

When you apply multiple coats of urethane, any dust or small flaws will be amplified with each new coat. If you want the smoothest finish possible, then wait until it is dry, very lightly sand with very fine paper (220 or finer) to remove any dust bumps, wipe it clean with a slightly damp cloth or tact rag, let it dry or buff it dry with a dry cloth, then apply the next coat. There is no reason to use mineral spirits, actually I would advise you not to use mineral spirits.

I see no advantage to applying successive coats with, then against the grain. As long as you apply an even coat with a good quality brush or foam brush and don't overwork it, any brush marks should disappear. The finished product should be so smooth that you couldn't tell which way you brushed the urethane on. Also, if you are using a satin finish urethane, be sure to mix it well, stirred, not shaken. The dulling agents tend to fall to the bottom, so it is important to mix it well. No matter how many coats you apply, it should not become "glossy".

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I generally spray-finish when I want to have a true satin sheen. I find that it's much easier to manage the application when I'm trying for a Satin finish, even if I'm just using a rattlecan, especially for the final coat. I might just suck with a brush, but I tend to find that I leave either brush strokes, tiny bubbles, or areas of uneven thickness that show up as a slight different variant of "satin" when I'm using a brush or a foam roller. I've since bought an HVLP gun and use that most of the time when I'm finishing doors and whatnot.

If you're using oil-based polyurethane (NOT the acrylic/"clean up with water" type) then you can use quad-ought (#0000) steel wool instead of 220 grit. I find steel wool easier to manage. If you are using the water-based finish, you can find "super-fine" 3M "finishing pads" in the sandpaper section. These, and a tack-cloth WITHOUT the mineral spirits (also usually in the sandpaper section), is what I would use between coats. Make sure that you're sanding in one area and painting in another, as the dust will get into the finish.

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0000 steel wool works good, I also like 320 or 400 auto finish papers. i like the spray idea, good points if you got a sprayer. Which gun do you have? love to see a link to it. –  shirlock homes Nov 15 '12 at 21:57
    
I have a Binks gun, can't remember which one and pretty sure it's discontinued. Briefly had an AOC (italian) gun before I returned it. Thanks to a few neighbors who have really cheap tools but are willing to help on projects, I've found that for just blowing a couple coats of polyurethane or oil paint (NOT Latex) over something, a $29 Kobalt or Husky gun works just as well as long as you have a viscosity cup and know how to use it. –  Karl Katzke Nov 15 '12 at 22:32

It's all about adhesion. When you recoat after 2 hours you get a chemical bond between layers. If you let it go longer than that, you need to wait 24 hours so it's hard enough to sand and get a mechanical bond. I sand before the final layer. That gets it smooth without danger of sanding through.

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Glossy or not, a smooth finish feels wonderful and protects better. Especially on something as abused as a table top. If those dust nibs you didn't sand out get knocked or scratched out at some point, it may result in a hole all the way down to the bare wood. Once moisture enters such a hole, well... you obviously have problems.

Assuming that convinces you to smooth out your finish, I think you'll get the best results if you apply in the direction of the grain, wait a day between coats, and always sand lightly with a fine grit in between.

Cue the TL;DR section.

Here's what works well for me. I like wiping on thinned varnishes for four reasons: it dries quicker, it's self-leveling as compared to full-strength, it wipes quickly without as much streaking, and cleanup is easier because you just let the rags dry and then toss them. Cleaning a brush involves solvents, gloves, a gooey sink, etc.

You can thin polyurethane 50/50 (or even a little more) with mineral spirits to make it easy to wipe. Minwax's Wipe-On Poly is also a good choice, though it's more expensive to buy pre-thinned. You need many more coats when applying it thinned, but the build is nicer in the end. A general rule of thumb is 3 coats thinned for every one you would have applied full-strength.

As far as 2 hours, I have little confidence in that... poly is usually tacky for quite a while (even if not obviously tacky to the touch). Thinning helps it dry faster, but it's safest to let it dry at least overnight. If you sand it before it's completely dry, marks will show up in the final result. Further, if the lower layer isn't completely dry and you seal it off with another layer, it will never completely dry (since it requires oxygen to cure). Ambient temperature and moisture in the air also affects curing time, but 24 hours is a safe bet in most climates.

Following the grain direction is a just-in-case technique that's useful to cover up application flaws. It's not necessary until the final coat, usually, but it certainly doesn't hurt to do it anyway. Especially if you don't have experience with brushing on varnishes; your technique is bound to be sloppy until you've done a few. This is not as much of a problem with wiping, but following the grain hides any streaking that might occur if your rag is too dry and you don't immediately notice. I always wipe in the direction of the grain on every coat because it's no extra work and I like to be on the safe side.

I love the feel of my finishes when I apply 3-6 thinned coats. After each coat, I wait a full 24 hours (I live in the cold, wet Northwest) and then sand lightly with 600 grit, remove the dust with a dry rag or vacuum brush, and do another coat. Sometimes I'll thin the top coat slightly more than the others, and then do a final sanding with 1500 or more so it hits any dust nibs but doesn't dull the finish. This regimen takes days of course, but if you have the patience your final product will be surprisingly smooth and appear very close to the wood.

Another rule of thumb: the more you sand the top coat, the more the finish will dull. This tip is useful if it comes out glossier than you wanted (#0000 steel wool or a super-high grit sandpaper is your best friend here). But to get in the ballpark, pick Satin if you want very little gloss and pick Gloss if you want more of a mirror look.

A caveat: this advice is based on oil-based poly and tight-grained woods; your mileage may vary if you're working on something with open pores (then you have to worry about wood fillers and other techniques for a smooth surface) or if you're using water-based poly (then you have to deal with grain-raising and all that).

A great book on finishing, if you're really interested in techniques, is Bob Flexner's "Understanding Wood Finishing: How to Select and Apply the Right Finish". One of the main themes is that the directions on finishing product containers are usually misleading at best. My comparatively limited experience has confirmed as much.

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Letting each coat fully dry is the most important thing. Temperature and humidity make a big difference in drying time. Don't assume the previous coat is dry; scratch it in an obscured place with a nail to test it. It should not feel soft.

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I always wait 24 hours between coats, and I usually do three coats. I use 00 grade steel wool in between coats to sand, then I wipe off with a cloth, and then use a tack cloth for any remaining particles. After the last coat, I'll either leave it as is, or if I want to dull the finish (for the most natural look you said you were going for), then I'll sort of buff it down with 0000 grade steel wool. I find this to be fine if you have at least three coats. I find steel wool a bit more dynamic than sand paper, and a little easier to use.

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