Question
I have been doing a few projects lately with high-gloss polyurethane finishes. The final coats keep having minor bubble spots and/or lint/dust spots. I have been doing it in a fairly dust free room to avoid most of the dust spots, but I still get a few. And I have been thinning my polyurethane, but I still get a few bubbles. I am assuming that they are hard to avoid entirely. So, my final coat is not mirror smooth. There are always some little imperfections and spots. I am primarily doing this on decent sized flat surfaces, 5-6 sq/ft. So, the spots are too noticeable for this type of project. How do you polish these few imperfections out of polyurethane after the final coating to still maintain a high-gloss, smooth finish? I have tried 2000 grit paper and 0000 steel wool. They are both way too abrasive, and dull the finish a lot. Also, do any of you use wax on top of your final finish?
Forum Responses
(Finishing Forum)
From contributor A:
You should use an automotive polishing compound and a buffer for big areas. Small areas will have to be polished by hand unless you have some funky buffing tools. Make sure the poly has cured for a minimum of 4-5 days at 70f. Do not try to polish an uncured film. Start gently, sand all the imperfections (wet 1000g) and then clean the surface well. Use a fine cut cleaner by McGuire's or 3M and follow the instructions on the bottle. Clean all residue when done with a damp cloth. Repeat the polish process with a swirl mark remover. Again, clean all residue with a damp cloth and buff clean with a cotton cloth.
Apply a final polish of your choice. This should give you a table that is like a mirror. Also, try this out on a test panel first to make sure you like the result.
Then there is the Mirka Abranet/Abralon series of abrasives. I went to their website and saw that the Abranet series abrasives are mesh and go up to 2000 grit. The Abralon series is foam backed and go up to 4000 grit (nothing between 2000 and 4000). I don't have a vacuum system hooked up to my sander, so I don't think the Abranet abrasives buy me much. I assume the Abralon would be the way to go if going with Mirka. However, I have been using 3M wet paper up to 2000 grit. 2000 grit still leaves the finish very hazy. 3M 2000 grit and Mirka Abralon 2000 and 4000 are all silicon carbide. Other than the foam backing on the Abralon, what is the difference? Will the Mirka 4000 grit be that much better? Or am I better off going up to 2000 with the 3M wet paper (4000 Abralon if I can find it), then following with Meguires type products? I have never seen Mirka products around. Where do you get them (I am near North Houston)? I do have a polisher. If I go up to 2000 grit (or 4000 if I can find it) and then I go with Meguires (or comparable 3M polishes), does anyone have a recommendation for which particular Meguires products (or 3M) to use? And in what order?
To avoid dust, are you wearing a Tyvek suit? Makes a lot of difference. In a spray booth that has lots of air flow, you would be surprised how much lint and dust the fan pulls off of you. Also, wet the floor down before you spray.
Sanding grits (wet dry): 500, 800, 1200, 1500. Then go to the polishing compounds. You will end up with a piano-like shine.
Comment from contributor T:
I recently completed a kitchen table project for my own family. We have three very active kids who can destroy pretty much anything you put in front of them. For the table finish, I chose a product called Ceramithane that I had used in the past on their nursery furniture. It has held up well because it is essentially a water-based gym floor finish that can be sprayed. However, it does tend to orange peel a bit and therefore required wet-sanding and polishing for my kitchen table project.
I sprayed on four coats with a light scuffing in between with 400-grit dry paper. After the final coat had cured for a couple of days, I started the wet sand process. I found a local auto body shop supplier with a good selection of 3M wet/dry papers. Using a stiff foam block purchased from the same store as the papers, I began with 1000-grit, liberally spraying the surface with water containing a little bit of dish soap (I used Dawn and it worked fine).
This first grit is the most critical as it flattens the orange peel and gets out the major imperfections. You have to let it dry completely before you can really see what you've got. You're looking for a uniform dull finish. Any areas that show up still shiny mean you haven't knocked them down enough and the finer grits will never touch them. Once you're sure you've got a good 1000-grit base to work from, proceed through 1200, 1500, 2000, and 2500 grits.
Keep the paper wet at all times - you're looking for a kind of milky "slurry" indicating material is both being removed and being carried out from under the paper. If your paper gums up with little globs of material, toss it and get another sheet. I found rubbing with the grain was fine, no need to make circles or figure eights. After finishing the 2000 grit pass, rinse off and let dry. The final step is to buff out the 2000-grit scratches with polishing compound. I used 3M "Perfect-It" and my Porter-Cable worm drive random orbit sander.
Both the rubbing compound and the foam pad that velcro’s to my sander came from the auto body supply store. Dribble some compound onto the surface and start up the sander. It's messy, so count on compound slinging around the area. Polish until the compound dries up, then wipe off any excess. You should see a near-mirror finish. Mine came out better than anything I've ever finished before in fifteen years of what I'll call serious hobbyist woodworking. It was much better than any sprayed or brushed finish. The only thing that might be better is a good French polish, but I personally don't have the time or patience for that. Good luck and one last piece of advice: practice on a test piece to get a feel for the papers - 1000-grit doesn't sound very aggressive, but it will eat your lunch if you don't know when to stop.
