Does “leveling” mean sanding with fine sandpaper (320 or better)? Does it need be done with a sanding block (as opposed to a short-stroke ROS)? Does this apply to all finishes (see question below) including polyurethane, both oil- and water-based?
I also see suggestions for “rubbing out” after leveling between coats – what does that mean?
And finally, I get very confused with many discussions because I do not understand the differences between major classifications like varnishes, lacquers, shellacs, polyurethanes, etc. Are these all mutually exclusive? Are there more major classes? Is poly the only one that can be either water or oil based? Is shellac (made from lac beetles) considered oil-based?
See – I said I was confused! But I am trying. I am reading the excellent Taunton Press FINISHING book by Jeff Jewitt but I still have these questions – maybe I missed something, but I think the problem is that he expects I will automatically infer the classifications.
Is there a useful link to answer my questions? Or a recommended FWW introductory article or book?
I hope my questions prove interesting even though they are basic. Thanks for reading them.
The Wood Loon
Acton, MA
Replies
I like Taunton's Complete Illustrated Guide to Finishing by Jeff Jewitt book. You should get many of your questions answered when you get to Part 5, which begins on p. 200. You can certainly jump there. It doesn't have to be read as a novel page by page.
To follow your questions in the order you present them: Leveling, should be done by hand generally with a sanding block--power sanding is much too vigorous, and round ROS sanders are particularly hard to really get a level surface. It's only appropriate with film finishes. Never any grit coarser than about 320. Cutting through the finish, particularly if there is a dye or stain can really add to your work. Leveling would be done on the finest work on any film finish. You can get rather good results with only very light leveling between coats. It can be done with all varieties but is easier on some than others.
Rubbing out is done only on the final coat, with the purpose of removing any defects that appear when that coat was applied, and also to adjust the sheen to the desired level. It's a process of moving to finer and finer abrasives until the abrasion leaves the desired degree of polish.
There are several categories that don't really correspond to the names. The first division is between evaporative finishes which are lacquers and shellacs, that depend of a solvent evaporating and leaving a resin as the finish. The is basically unchanged chemically and can be redissolved by the same solvent.
The other broad classification is that of reactive finishes, these are the varnishes and also include the subclassificatin of coalescing finishes which include the waterborne finishes. Out side of the range of amateur finishing are finishes similar to lacquers that have a catalyst added to create a reaction, or other two part finishes where combining the parts kicks off a chemical process that creates the finish resin.
Polyurethane is just one variety of resin, not a whole class of finish, except in the lanquage of marketers. In consumer grade finishes it is combined with another resin, generally an alkyd resin. Adding polyurethane to the resin does add abrasion resistence. Varnishes without polyurethane, oil based varnishes such as alkyd resin or phenolic resin varnishes are virtually as protective as the single part varnishes with polyurethane. The dominent resin in waterborne finishes is acyrlic, and again polurethane is sometimes added for added abrasion resistence, or for marketing reasons. There are hundreds, perhaps thosands of different polyurethanes, each with different properties, so generalizations have to be couched carefully.
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