Popular Woodworking 2008-06 № 169, страница 64

- Flexner on Finishing -

BY BOB FLEXNER

Rules to Finish By

They explain so much.

t's often possible to sum up a lot of situations with a rule, a principle that applies in almost all cases. When I teach seminars on finishing, I often find myselfcitinga rule I've created to explain a procedure or to answer a question. These rules can be very helpful for understanding finishing.

Here are my five favorites, the ones 1 repeat most often.

Rule#1

Choose a grit sandpaper that removes the problem efficiently without creating larger-than-necessary scratches that then have to be sanded out.

This rule answers the question, "What grit sandpaper should I use?" It varies for differ-

For example, you would choose a coarser-grit sandpaper (#80 or #100) to remove severe washboarding caused by a jointer or planer but a finer grit (#120 or#l 50) on pre-sanded, veneered plywood or MDF. And you would begin sanding with an even finer grit (#180 or #220) ifyou were just checking to make sure your stripper had removed all the old finish from a refinishing project.

Likewise, you would choose a coarser-grit sandpaper (#220 or #320) to sand out brush marks in a finish but a finer-grit (#400 or #600) to remove fine dust or orange peel.

It's most efficient if the grit you begin with isn't any coarser than necessary so you don't have to sand out the deeper scratches.

How do you determine which grit is appropriate? Experience is the best teacher. In the meantime begin with a grit you think is about right, or even a little finer than necessary and then "cut back" to coarser grits until you find the one that removes the problems efficiently. Woodworkers will disagree here because everyone sands differently. But keeping this

principle in mind will help you reduce the amount of work.

In all cases, remove coarser-grit scratches with finer grits until you reach the grit you want to end with. Skipping a grit will require you to sand more to remove previous grit scratches than progressing through each successive grit, but either way is legitimate.

Rule #2

There are only three common tools used to apply finishes: a rag, a brush and a spray gun (including aerosols).

Finish application is therefore far less complicated than woodworking, with its dozens of tools. Each of the three finish tools transfers liquid - finish, paint, stain, whatever - from a can to the wood and is simple to use. Even a spray gun is no more difficult than a router.

On large surfaces, fast-drying finishes are harder to apply with a rag or brush because you can't move fast enough to keep a "wet edge." But there's less problem if the surface

is small-for example, a turning. All finishes, no matter how fast or slow they dry, are easy to spray onto any surface.

The real differences in the tools are cost, speed and the degree to which they produce a level film.

Rags are cheap and efficient for applying any stain or finish you intend to wipe off, but they leave pronounced ridges in finishes when you're trying to build a film.

Brushes are also cheap and are the least wasteful of finish material, but they're very inefficient because they transfer the liquid so slowly, and they leave ridges (brush marks) in the film.

Spray guns transfer the liquid very rapidly and leave the most level film. But they, and the added compressors or turbines needed for operation, are expensive. And because of the finish that misses or bounces off the surface, spray guns are wasteful of finish material and require an exhaust system, which increases the expense.

Three tools. Compared to the complexities of woodworking, finishing is easy. Most importantly, there are only three categories of application tools: rag, brush and spray gun.