Sanding "too" fine

Hey folks,

I'm a complete newb to both the forum and woodworking. I built up a set of speakers some time back and finished them with a near piano gloss mahogany veneer. I'm building another pair in birch ply and want to A) use a stain followed by clear polyurethane or B) a colored polyurethane to achieve a piano gloss effect.

I'm doing up some test panels at the moment and sanded some plywood to 1200 grit. I rollered on colored poly and it wouldn't adhere properly at all. Now, I had read on another forum that to get a piano gloss finish you need to start with 1200 grit, but that poster had sprayed the polyurethane rather than brush/roller.

So, is anything above 400 grit too much for the base layer and leave the finer grits for the finishing layers? I doubt a stain would adhere properly to a surface sanded with 1200 grit either.

Any experienced input would be much appreciated.

Some would say that sanding bare timber beyond 240 grit when using a high build finish that sits on top of the substrate is a complete waste of time, I'm one of them. The finer grits come after the initial coat of finish. When you say 'piano gloss finish' I guess you mean a finish that is totally devoid of 'features' in its smoothness and with a very high gloss level. The best way to achieve this with polyurethane is to wipe it on rather than roll or brush it. You'll almost certainly need multiple coats until none of the irregularities of timber grain can be seen/felt, sanding with progressively finer grits up to your aforementioned 1200 grit.

Coloured/tinted polyurethanes are incredibly difficult to get an even finish with in my experience so probably best avoided. Polyurethane may not even be the best product to achieve the finish you're looking for. If you have a look for online advice from luthiers as to the finishes they use you may find it useful.

Welcome to the forum. I love your choice of user name, it's amusingly self derisive. Surprising we don't have wreckers numbered 1 to 100 on here, I'd probably be about number 32. :wink:

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Originally Posted by Wrecker

I'm building another pair in birch ply and want to A) use a stain followed by clear polyurethane or B) a colored polyurethane to achieve a piano gloss effect.

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Hello and welcome to the forum. I recently got back from 3 weeks driving around Ireland. We had a ball.

I associate "piano gloss" with a mirror finish. If that's what you are aiming for, you won't achieve it with one-part polyurethane. They are a bit too soft to polish. However, you can achieve a nice shiny surface that will give a little reflection with one-pack poly.

With very high gloss finishes the shine is achieved by wet-sanding the finished surface (after at least a week of hardening) with wet & dry sandpaper, then polishing compound. You typically start with 600 grit and work up to 1500 or 2000 grit, then polish with compound.

I think 240G it a bit too course for the pre-finishing sanding. Particularly of soft birch ply. I usually go to 320 grit.

Be carefull staining the ply before applying the finish. It can turn out blotchy if you aren't carefull. It's better to dilute the stain with a reducer and build up the colour in stages.

Really, the best grit to stop at depends on how fine the timber grain is. I generally stop @ 240 on most timbers but the occasional piece gets up to 320 or, rarely, 400.

If you think of timber grain as bundles of straws adhered together, then once you work down to a grit size that's close to the diameter of these straws that's pretty much the stopping point. The overall surface is as flat as it gets and you can't sand out any open grain and/or pores without opening up even more, so further grits accomplish little beyond taking up your time & resources.

If you're thinking of stopping but can still see scratches across the grain, try using the same grit to give a final sand with the grain to get rid of 'em.

Should you work even further down the grits you're no longer really sanding, what you're doing is burnishing the wood. ie. collapsing the sides of the straws in on themselves - 'filling' them as it were - giving any later finish less to key into.

So yeah... save the finer grits for the finish itself.

Wrecker

Welcome to the Forum.

You pose an interesting question. My rather simplistic take is that finishes, whether they be paint or varnish need to grip onto the surface. While different treatments will require slight modification, the principle is fundamentally the same.

A long time ago repainting a sound paint surface needed merely to be rubbed lightly with medium grade sandpaper. I don't really know to what grit that would have equated, but I suspect not finer than 240g. With bare timber, as aldave and Skew have pointed out, little is gained by sanding beyond 240g or perhaps up to 400g depending on the timber: They don't all behave the same by a long stretch. The first coat of varnish is a sealer and can be applied slightly diluted to aid penetration. Then add successive coats to build up the finish, but it is important to sand between each application for two reasons. The first is to key the surface for the next coat. The second is to even out any undulations.

In fact, if you follow Jack's recommendation using Wet & Dry (used wet), the aim will be to sand and recoat until the sanding reveals a completely flat surface (flat as in matt finish). The presence of small, slightly glossy dimples means the surface is not geometrically flat and the finish cannot be the best possible. When sanding you should avoid rubbing in the same direction as your fingers. In a fine finish it will result in finger lines. Sanding with a cork block will help there.

As far as a piano finish is concerned, I believe it references the type of surface found on a grand piano (and possibly any French polishing too). "Piano" finish has been around for a long time. Arguably the finest hand saw ever made, the E.C. Atkins No.400, (more info here if you fancied heading down the hand tools rabbit hole) had a Brazilian Rosewood handle with a "Piano" finish .

This is from a 1919 catalogue:

Attachment 534077

My own example, not really displaying the "piano" finish to the best advantage, is approaching 80 years old. The very first models were pre-WW1.


Attachment 534078

My recommendation is to stick to speakers. :wink:

Regards
Paul

To produce a high gloss finish depends largely on the density and size of the pores of the wood involved. For a wood similar to birch, I would sand the bare wood up to about 400#, The apply a stain of choice, allow to dry thoroughly and sand with 400 or 600#. Repeat two or three times or as many necessary to get an even colour, sanding between coats. This requires some judgement, and is a rather messy process. Then apply some tinted sanding sealer again two or three times or until there is no evidence of a powder lodged in the grain, with 600 0r 800#, preferably wet & dry paper, used dry, again this is a messy process. Then apply an even wet but thin top coat. Sand with 600 or 800#, but this time wet with water. Continue applying thin coats of the top finish and sanding with up to 1200# W&D paper used wet. Lightly wipe the paper over a dry cake of soap may help getting a finer finish, until a satisfactory finish is achieved.
It is imperitive that all materials are compatable, whether the materials are water or spirit based is a personal choice, but do not mix them. Check the U Beaut catalogue, you will find all you need there. All UBeaut products are compatable. Personally I prefer spirit based materials and use Mirotone products. My personal preference for a top coat of precatalised lacquer at 60% gloss and sprayed. With some practice, the lacquer can be applied with a rubber similarly to french polish. A final buffing with EEE wax will improve the surface finish.
Hope this helps you achieve your goal.

Jim

Quote:

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Originally Posted by Wrecker

I'm building another pair in birch ply and want to A) use a stain followed by clear polyurethane or B) a colored polyurethane to achieve a piano gloss effect.

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Firstly, Top O' the Mornin' to ya!, In my experience Birch doesn't take a dark stain too well, especially if its a solvent based one. Me personally I'd seal the birch with shellac first. Then add a dark tint to the poly and wipe it on. keep doing that until its dark enough for you. For the final coat add a pretty thick coat of clear poly. If you add a thin coat then you will burn through it when you goto rub it out to a high gloss and you'll get witness lines, multiple coats of poly do not melt into one another like for instance shellac / 2pac / waxoil.

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Originally Posted by Wrecker

I'm doing up some test panels at the moment and sanded some plywood to 1200 grit.

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Are you using the same birch plywood for your test pieces?. When you say its not adhering, what's it doing?, blooming, beeding?...

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Originally Posted by jack620

Hello and welcome to the forum. I recently got back from 3 weeks driving around Ireland. We had a ball.

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Happy days. Shame the weather was complete !

I had pretty much figured out your suggestions before reading them and there will be plenty of test pieces before attempting the final product. I also realised I had forgotten to thin the poly before applying which would have helped it adhere better.

I'll take another crack at it with lower grit paper then stain and a few coats of clear poly over the top. I'll be sure to post a pic of the final results.

Thanks to everyone that read and replied.

Michael.

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Originally Posted by jack620

Be carefull staining the ply before applying the finish. It can turn out blotchy if you aren't carefull. It's better to dilute the stain with a reducer and build up the colour in stages.

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Thanks christ for test pieces. A bit of deeper reading said Birch must be treated with a pre-stain conditioner as it soaks up stain unevenly and quickly, which I can attest to.

Now I understand why the tin said 6-8 hours to touch dry and the piece in my hand was touch dry in less than 5 minutes! :no::no:

Hey folks. Thanks for the input thus far. I have treated the test piece of birch with pre-stain conditioner and the first coat of stain went on fine, no blotches. Not so with the 2nd coat.

Should I been sanding between coats of stain?

Quote:

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Originally Posted by Wrecker

Hey folks,

I'm a complete newb to both the forum and woodworking. I built up a set of speakers some time back and finished them with a near piano gloss mahogany veneer. I'm building another pair in birch ply and want to A) use a stain followed by clear polyurethane or B) a colored polyurethane to achieve a piano gloss effect.

I'm doing up some test panels at the moment and sanded some plywood to 1200 grit. I rollered on colored poly and it wouldn't adhere properly at all. Now, I had read on another forum that to get a piano gloss finish you need to start with 1200 grit, but that poster had sprayed the polyurethane rather than brush/roller.

So, is anything above 400 grit too much for the base layer and leave the finer grits for the finishing layers? I doubt a stain would adhere properly to a surface sanded with 1200 grit either.

Any experienced input would be much appreciated.

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What is the specific problem you are having? "Did not adhere properly" could cover an ocean of problems.

For example, did it peel back off? Did you get fish eyes or big dips/low spots? Blotches where it soaked in some places and light spots in others? Did it fail to cure and stayed gooey?

This will help us point you in the right direction.