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Winter 2013-2014

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Super-Smooth Poly Finish

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Super-Smooth Poly Finish

A defect-free finish, even with a brush

By Eric Smith


Polyurethane is a tough, high-quality finish, ideal for tabletops and other surfaces that take a lot of abuse. But no matter how clean your finishing area or how good your brushing technique, a few bubbles, dust particles and streaky spots always manage to sneak into the final coat (Photo 1). Directions on the can don’t say anything about it—leaving you to assume a less-thanperfect finish must be your own fault. However, the solution is quite simple—rub out the finish with fine sandpaper and synthetic steel wool. Sanding removes defects and levels ridges. Synthetic steel wool creates an even, silky smooth finish that’s a joy to look at and feel. This age-old two-step technique is commonly used on shellac and lacquer finishes, but it can work well on water- and oilbased polyurethane, too. The only drawback with poly is that it is difficult to bring up to a high gloss. If a satin or semi-gloss look is what you’re after, this technique will give you great results.

 

Materials and Supplies

Sandpaper

Sandpaper is used to flatten the finish and remove dust nibs and brush marks. Stearated aluminum-oxide sandpaper is by far the best product for sanding a finish. Stearated paper has dry lubricants that help prevent “corning” or the balling up of finish on the paper. Wet-dry silicon-carbide paper balls up like crazy if you don’t use water as a lubricant. The trouble with wet sanding is the water slurry can make it difficult to see your progress.

Synthetic Steel Wool

I used synthetic steel wool on both water- and oil-based polyurethane. Traditional steel wool is not recommended for water-based finishes; it sheds steel particles that leave a mess and give the user steel wool slivers. Synthetic steel wool pads equivalent to 00 steel wool are widely available at home centers and hardware stores. Fine synthetic wool equivalent to 0000 steel wool is harder to find. I had good luck at auto-body supply stores and mail-order woodworking suppliers (see Sources, below).

Powdered Abrasives

Pumice and rottenstone are sold at some paint stores and at woodworking suppliers. Pumice is ground volcanic glass that comes in grades from 1F (coarse) to 4F (fine). Rottenstone is even finer than 4F pumice. It’s made of ground limestone (see Sources).

 

Build a Good Foundation for the Finish

1. I use 220-grit sandpaper for final sanding on raw wood. I always sand a little bit longer than I think is necessary. Then I vacuum thoroughly and wipe the wood with a clean, soft cloth until I stop getting dust on my fingers when I run them over the wood.

2. Use grain filler on open-pored woods, such as oak or walnut. Otherwise after rubbing out, the pores will look shiny compared with the rest of the wood.

3. Before applying finish on any project, test different finishing options on scrap pieces of wood. Water- and oil-based polyurethane finishes look completely different. If the color doesn’t look right or seems too bland, which is sometimes a problem with water-based finishes, use a sealer coat of clear, wax-free shellac or experiment with stains to warm the color of the wood before applying the topcoats.

4. I applied a gloss polyurethane on my tabletop because it can be rubbed to any sheen from flat to semi-gloss. I used a semi-gloss poly on the rest of the table. Vertical surfaces and legs don’t collect the dust the way a flat, horizontal top does. A light buffing with steel wool will clean the occasional dust nib on vertical surfaces.

5. Sand with 320- to 400-grit stearated paper between coats, depending on how smooth the coat looks. Use a sanding block to level ridges and bumps. With a gloss finish, coarser paper may leave scratches that are visible through subsequent layers of poly.

6. Apply an extra coat or two of polyurethane on tabletops for more durability, depth and protection. Lay the last coat on a little thick to protect against accidentally rubbing through the top layer of finish. Remember, polyurethane does not melt into itself the way shellac or lacquer do. Each layer sits on top of the previous one, so there is a danger of sanding through one layer into the next. This will leave a visible ghost line where the top layer was sanded through. If this happens, you need to reapply the last layer of polyurethane and start over.

7. Finish the test boards at the same time you’re finishing your tabletop. Use these sample pieces to make sure the finish is properly cured and ready to rub out. Then experiment on them to get a feel for rubbing out.

8. Let the finish fully cure! This is most important for a successful rubout. A finish that has not cured will not be hard enough to take an even scratch pattern from abrasives. The result will be an uneven sheen. Polyurethane should cure for two weeks to a month after the last coat is applied. If the finish balls up on the sandpaper or it won’t buff out to more than a satin sheen, let it sit for another week or two.

 

Smooth and Flatten the Finish

It seems completely counterintuitive, but to make a finish really shine, you have to start by sanding it dull (Photo 2). Sanding removes dust nibs and brush marks and leaves the finish smooth and flat.

Caution: Finish tends to be thinner at tabletop edges. Use special care in these areas to avoid sanding through (Photo 3).

9. Apply consistent, light pressure as you sand. When you’re done, the surface should feel smooth and level and will still have a few small shiny spots. Don’t feel that you have to completely erase every visual defect at this point— just go for a smooth feel. Unless you have lots of bubbles to flatten, you should only need to sand five to 10 strokes in any given area with the 600-grit sandpaper. Sand dry so you can see what’s happening to the finish, and change paper often. Vacuum all the sanding dust off the surface and wipe with a damp cloth. Tackcloths can be used on oil-based poly but not on water-based.

 

Rub to an Even, Flat Sheen

10. Begin rubbing-out with medium-grade, (00 steel wool equivalent) synthetic abrasive pads (Photo 4). This is where the finish begins to come to life, taking on an attractive, flat sheen with no visible defects.

 

Rub to a Satin Sheen

11. Clean the top with a damp cloth and continue buffing with fine synthetic abrasive wool (0000 steel wool equivalent) (Photo 5). Rub until the whole piece has an even, satiny sheen, and then rub a little more. There’s not much danger of rubbing through the finish at this point.

 

Rub to a Semi-Gloss

12. To bring up the sheen even more, use soapy water or paraffin oil as a lubricant for the abrasive wool (Photo 6). Rub thoroughly; then wipe dry.

13. If that’s still not enough shine for you, rub the entire surface with 4F-grade pumice. After sprinkling the pumice on the surface, rub it into a paste with water and a dampened rag (Photo 7). Wipe the slurry away, and then repeat the process with rottenstone. Keep firm pressure on the rag, and sprinkle more of the powder or water as needed. Continue rubbing in any direction until your arms hurt and the finish looks satisfactory. Now your furniture has the goodlooking finish it deserves.

 

Sources

Note: Product availability and prices are subject to change.

Home centers and hardware stores, 3M packs of two finishing pads, 00 steel wool equivalent; Sandblaster 400-grit stearated aluminum oxide paper.

Woodworker’s Supply, woodworker.com, 800-645-9292, Oilfree abrasive wool, fine (000 to 0000 equivalent), #115-271; medium (1 to 00 equivalent), #115-274; 4F pumice stone, 1 lb., #849-832; Rottenstone, 1 lb. #849- 839.

Click any image to view a larger version.

1. The Problem: A few dust nibs, broken brush bristles and bubbles are almost inevitable on big horizontal surfaces finished with slow-drying polyurethane.


2. The Solution: Flatten the surface imperfections with 600-grit sandpaper on a sanding block (or 400-grit followed by 600 if the surface is really a mess). Sand just enough to flatten bubbles, dust nibs and ridges, but don’t try to sand away all the shiny spots.


3. Extra care should be taken when sanding near the edges of a tabletop to avoid sanding through. Sand the 2 to 3 in. nearest the edge first. Short strokes make it easier to control the block. After the edges are done, sand the centers with long strokes that overlap the sanded border.


4. Rub out the finish using a medium synthetic abrasive pad (00 steel wool equivalent). Rub until you get a flat, even sheen across the entire surface.


5. Switch to a fine synthetic abrasive wool (0000 steel wool equivalent) to bring the finish to a satin sheen.


6. For a semi-gloss sheen, continue rubbing with fine synthetic abrasive wool lubricated with soapy water.


7. Using finer and finer abrasives brings the sheen closer to a full gloss. Start with finest-grade (4F) pumice lubricated with water and a moist rag, followed by rottenstone. With these finer grits, it’s OK to use a circular motion as you rub.


Dealing with Molded Edges

Avoid using sandpaper on molded edges, table legs and other vertical surfaces. The risk of cutting through the finish with the sandpaper is just too great. Instead, rub molded edges with synthetic abrasive pads and rub to the sheen of the top.




This story originally appeared in American Woodworker September 2006, issue #123.

September 2006, issue #123

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