How smooth is smooth enough? Amateur Q.

I'm a stone-cold amateur at woodworking. Doing my very first project of any kind: making a simple desk out of some plywood and electrical conduits for legs.
I am sanding the plywood (birch) and I realized that I don't know when I am done... I know that I am going to add polyurethane after this, but am unclear whether that is *only* for keeping moisture out... or if that is also the final stage of "smoothing."
Do I sand until the surface of the desk is perfectly smooth and then add the polyurethane? Or is mostly smooth okay and then the polyurethane makes it smoother?
In general, how smooth can I expect to make the surface of my desk? Can I get it as smooth as the ones in Home Depot? Or are they that smooth because of some other material/procedure?
thanks for any help! scott.
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"m. scott veach" wrote

With plywood you'll know all too soon ... right after you've sanded through the veneer.
With most furniture grade plywoods, a _light_ sanding with anything from 150 to 220 grit should suffice for just about any finish.
For your particuar project, take a piece of the plywood scrap and sand away at it until your break through the veneer. This will give you a practical idea of what to expect with that particular material.
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Swingman wrote:

Applying a sanding sealer before sanding can make a smooth finish easier to get with far less wood removal.
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To add to Swingman's advice you can get it as smooth or smother than the Borg stuff. Between each coat of Poly or after the first couple hit the surface with #00 steel wool (Only if using oil-based poly). Then a few weeks after the finish has cured you can buff it out with #0000 steel wool.
I would also rig up a way for the top to be vertical while applying the finish. This will decrease the amount of dust that settles on the surface. Simply hanging it by the conduit brackets is OK but it will be tough to apply finish on a swinging target, BTDT.
Forget the brushes foam or otherwise, use old t-shirts or buy a bag of cut-offs.
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I understood what you said, and why, and agree with why you said it. But it needs a clarification, IMO and leaves out way too much for a newbie. To coat something vertical you just about have no choice but to use a wipe-on method of application, not a brush of any kind, for a table-top smooth finish. Foam, bristles, etc. are very likely, almost sure, to result in runs, especially for a neophyte. Wiping the finish on can be tedious and isn't for the first timer with his final product.
Urethanes et al are "self levelling", meaning that they are forgiving for brush marks, overlaps, etc. because they will let gravity smooth them out when they're applied to a horizontal surface per the instructions that come with them. With a good brush and a new can of urethane, it's possible to get a very smooth, professional looking finish IFF the dust can be kept under control. Several thin coats are also much better than one thick coat. I usually use 3 coats, sometimes 4, depending on what it is and how deep I want the gloss, if I'm doing a glossy. Personally I like semi-gloss better but that's a matter of preference. For dust control I have a celing fan with a furnace filter attached to it. I turn it on a couple hours before I start and leave it on during the application. Then I avoid creating more dust with other projects during the setup times and my last coat comes out perfect for me. Horizontal storage during the setup time is easier too since they can just be placed flat on various surfaces. Once the material sets up, usually about an hour, it's OK to store them in any position as long as painted edges etc. are kep from touching anything.
Everything you said made good sense; it's just the missing info that I wanted to add.
Regards,
Twayne

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SNIP

Always looking for ways to control the dust, but can't imagine how you attach the furnace filter to a ceiling fan. Can you please "draw" me a word picture.
Thanks, Gary
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I'll try, but I can send a pic or two if this doesn't work. It's really pretty simple. AH, I see! Not a "celing fan" but a fan ON the ceiling. Sorry if I made that confusing!
I have a 20 x 20 box fan suspended from the celing so it can be aimed from near horizontal in either direction or straight down at the work, especially the table saw to push stray dust out of my way when I saw. For finishing, I use a 20 x 20 furnace filter and just tape it over the "in" side of the fan. I use paper shipping tape bacause they get dirty pretty fast at first. I run the fan a medium speed. I'll also vacuum off the dust a couple times until the air starts getting clear, then when I'm ready to finish I put a new filter on it. They only cost about a buck apiece at Lowes; same ones as my house furnace uses, in fact.
When the fan is running the filter will stay in place without any tape, but you do have to tape it so the filter will stay in place when you turn the fan off.
No, it's never harmed the fan and it pulls the air thru the filter just fine. When I'm not doing finishing I'll usually aim it straight down and toss a 10 x 10 on it for GP's of catching the dust. Keeps the shop overal cleaner it seems.
Twayne
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Twayne wrote:

It is also much easier to control dust if you use a water based finish simply because the surface dries to touch must faster than any oil based finish. This gives dust much, much less time to stick.
John
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After sanding, run your fingers across the surface. If it feels to you to be as smooth as your wife's ass you're ready to apply finish. If it feels as smooth to you as my wife's ass, watch your back...
--
FF



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unless your wife weighs 300lbs and has pimples on her ass. If she does - keep sanding.
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-Mike-
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the wood or the wife?
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On Fri, 11 Jan 2008 19:06:48 -0500, "Mike Marlow"

Her ass? <G>
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"Mike Marlow" wrote

Confucius say: "If wife's ass feels smooth, you need new girlfriend!"
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Metal tube legs joined to a plywood top? That's HARD. Wood legs joined to a skirt, with the skirt clamped to the top, is much easier. Trying to mate metal to wood only works well (without squeaking and fragility) if the metal is a flat plate.
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The best deal I've found for making a table with metal legs is to use "Curry Legs" from Ikea. For something like ten or twenty dollars you get a really nicely painted set of sturdy metal tube legs, complete with flanges for attaching to wood. What you attach them to is up to you. I've got two very functional computer tables made out of scrap interior doors with a couple of 1/2" plywood strips glued to the underside for strength. The legs are attached to the plywood. I'd recommend these over electrical conduit any day.
- Owen -
wrote:

Metal tube legs joined to a plywood top? That's HARD. Wood legs joined to a skirt, with the skirt clamped to the top, is much easier. Trying to mate metal to wood only works well (without squeaking and fragility) if the metal is a flat plate.
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I've made a couple of desks in the past. The first one wasn't smooth enough. When you write on it, the grain shows thru to the paper. So I learn from that to use a piece of paper and a #2 pencil to check for smoothness. Put the paper on the desk and, using the side of the lead, shade an area of the paper. If it comes out evenly shaded, you're done. If the wood grain is obvious, sand some more.
And yes, the polyurathane will smooth things out if you can avoid runs and drips. But a smooth starting surface helps a lot and requires fewer layers.
And as others have mentioned, be careful about sanding thru the plywood.
--

Mike McDonald
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