Using Polyurethane

[Joe Von Kaenel]

Hello,

This my first project using a polyurethane finish. I have been comfortable with and always used Tung Oil. When using Polyurethane is it advisable to wipe the entire project with #0000 steel wool prior to applying the final coat? I want a satin finish. Thanks

Joe

[Prashun Patel]

If you want a satin finish, you can either use satin poly, or you can rub down the finish with 4O steel wool AFTER the last coat. But Poly dries a little softer than other topcoats. This means it's possible to rub DOWN the gloss, but harder to rub it up from a satin to a gloss.

It is advisable to rub poly with a mild abrasive (320-400gt sandpaper or synth steelwool) in between coats. With poly it does 2 things: 1) provides more surface area for the top coat to bond, 2) assists in the proper flow and leveling of subsequent coats. I wouldn't do this abrasion with real steel wool. Unless you are extremely careful and use a magnet to get all the wool off, and then vaccuum, then you risk leaving grains on the surface, which will seal under the topcoat and ultimately rust (since no topcoat is completely impervious to water vapor).

[Jim Becker]

When you say "tung oil" do you mean something labeled "100% pure tung oil" or something labeled "Tung Oil Finish"? If it's the latter, it probably has no tung oil in it and is merely a wiping varnish. The former would be a penetrating oil.

If you are asking about oil based polyurethane varnish, it's just varnish with some abrasion resistance added due to the polyurethane resin added to the other resins used to make the product. It will apply like any other varnish with the exception that it needs to be lightly abraded between coats to insure that each coat will adhere well to the previous. Shawn talks about this very clearly.

If you are concerned with getting a good result with brushing, you can thin the varnish 50/50 with mineral spirits (paint thinner) and use it as a wiping varnish. Figure about 3 coats to equal one brushed coat, but application is easier.

You might want to consider a non-poly varnish, too. There are some great choices out there, although you will unlikely find them on the home center's shelves. Pratt and Lambert #38 is a good example. Wonderful product. Check out what the "real" paint stores in your area have available!

[Stan Urbas]

we usually just take a small block of hardwood - maple or oak - about the size that comfortably fits in your hand - and rub it down with that. You just want to knock down the fuzz from the prior coat. No need for sand paper.

[Steve Schoene]

With varnish, and particularly with polyurethane varnish, it is quite desirable to use sand paper (320 grit is ideal) between coats, particularly if you ever let your time between coats stretch to more than just over night. Such reactive finishes need the "tooth" of the sanding scratches to promote good adhesion between coats. You can get the adhesion with steel wool but not the leveling and clean nib removal that you get with sandpaper on a block.