"Mirror Finish" on Urethane Wood Coating

you maybe pushing too hard thus heating up the surface too much, have you tried something like duroglit? It´s in a can and you pull out a wadge and polish by hand! Or maybe even use a car polishing mop, not a disk, they tend to be more user friendly. I would start with the disk to get it smooth then go on to the mop to polish!

How bout an orbital buffer. Not a grinding wheel.

Paste wax and polish by hand!

progressivly finer sandpaper along the grains between coats, 1 coat 220, 2nd coat 400, 3rd coat 600, fourth coat 1000, and final coat 1200 grit or finer. factory applied urethanes can be even up to 7 coats or more! second, atmospheric conditions affect the "laying" of any coating. low humidity, <10% and moderate temperature between 65 and 70 degrees. dust free environment and slight thinning of the urethane, no more than 15% but not less than 7 or 8%, should do the trick. by the way, the surface being coated also needs to be horizonal for such perfection.

An old mold maker trick for getting to a mirror finish. After getting all marks out with progressivly finer paper (1200 or finer as another has already correctly suggested) polish with Simichrome polish applied with Charmin toilet paper. Yes, toilet paper. Charmin is the only one that has no wood bits in it which would scratch your work.

finnish it as you would a car with progressively finer sand paper wet sanding surface. They make a 2000 grit for cars. Then use car compound to finnish

One thing though the urethane takes longer to harden the car paint.

I would wait until a couple weeks to let it dry and harden before finnishing.

Would help to thin urethane with fast evaporating thinner and appling multible thin coats so it hardens quicker. Maybe adding a hardener.

I get fairly good results with dipping my bare hand in boiled linseed oil, and then, while the hand is still wet with the oil, drizzle urethane spar varnish onto the palm of my hand. This might take three or four coats, with the first coat more oil than varnish, and the last coat a little more varnish than oil. The warmth of your hand causes some magic. It would cover the swirl marks you complain about in one application!

It goes quicker than you think since the oil causes the varnish to harden up within 10 minutes, and any dust after that point wont stick. You can feel the goop start to "grab up" after only a few minutes of rubbing, and then you can stop, and let the gloss come up. You should let it harden overnight between coats though. A light going over with a cabinet scraper between coats will knock off any dust that does get onto the surface.

I have been known to actually add some dust for the first couple of coats just to fill the pores. Another trick is to prep the bare wood with a wet rag. Wipe it on, let it dry overnight, and scrape the little fuzzy bits off with a cabinet scraper. Then go to the hand rubbed finish above.

I like to use semi-transparent stains like Minwax Golden oak instead of plain oil to lend a nice rich finish and to make the grain pop out. When I use stain, the first coat has no varnish in it at all. The result is a gun stock which looks like it has been dipped in glass.

Darn! I wish all these questions were so easy to answer! Try it on a piece of scrap first okay?

No one who has responded has talked about sealers. The best gloss on wood can usually be had after sealing the wood to fill the pores and give a clean, smooth surface. You then want several thin coatings of PU finish, drying between coats, and a very light sanding (for power sanders, use a pad, not grit) between coats. Are you using oil based or moisture curing PU? The solvent in the coating is also a factor, as are surface flow additives and bubble release agents. Most manufacturers use mineral spirits in oil based PUs and xylene plus a small amount of active solvent, such as cellosolve acetate or butyl acetate, in moisture cures. Check out additives such as Byk 054 for starters. About 0.05 to 0.1 % by weight on the PU. This reduces the surface tension and allows for a very smooth, clean film. Are you using aromatic or aliphatic urethane finish? Are you adding your own catalyst? If oil modified, the finish will scratch rather easily. What is the hardness (Sward, for example) of the finish? Most importantly, you have to do the coatings in an area completely free from dust and any airborne contaminates. Remember, it's an art, not a science!!

steel wool and thin down the varnish

and it will look and feel like glass

Mirror finish

The proper way to do this is to seal the wood with a urethane sealer - likely solvent based

then apply multiple coats of automotive clear coat (2K urethane) sanding between coats and "cut" polish the final finish

I know a guy who finished piano's (piani?) professionally, ending with an optically flat mirror finish in any color or opacity. He used a high volume low pressure two part spray of catalyzed epoxy in a clean room with laminar air flow filtration. He would polish between every coat, wet sanding with an orbital sander to 1500 grit. Like others have said - it's an art not a science - many of the suggestions listed in this stream of responses will likely give you great results - each with its own peculiar combination of drawbacks or weaknesses. I think the place to start is what is the object to be finished - and is urethane even the right material for it?

OK, some of you are close but in finishing close does not count.

First if you are going for a mirror finish, you must start with the bare wood prepared properly.

Wood>>> sand to 220 grit >>> sand seal (thin) >>> 220 grit >>> squeegy a thick fill (clear or stain) this fills and levels the surface >>> Cure for 4 days (Room temp). Now DO NOT USE URETHANE FOR THIS OR VARNISH USE ONLY Sweetish oil, tung oil, or good boiled linseed with tong oil>>> sand to 600 grit >>> second sand seal >>> 1 day stand>> 800 grit >>> Clean surface with tack cloth >>> apply first coat of oil/tung combination >> let cure for 1 day >>> sand with 00 steel wool, Repeat over 30 day period at least 5 to 8 coats, progress to 000 steel wool, THEN use CARNUBA WAX, at least 3 coats by hand, buff with buffer.