Sanding: A Key Part of Finishing Prep

If there’s one thing finishers agree on, it’s that finishing prep is one of the most important steps to achieving a professional finish. But it ranks on the fun scale right there with toothaches.  In this article I’ll give you my hard-earned “near-painless” sanding prep procedures. Let’s go!

Pre-sanding Work

There are a few important things you can do early on to make your life that much easier:

 

Hand plane or scrape off all machined surface marks prior to joinery.

Pro Tip #1: Learn how to use a hand plane and scraper (or scraper plane). You’ll be amazed how much prep time these tools save you – even on curved surfaces.

 

Limit glue squeeze-out by learning how much glue to apply, and where.

Pro Tip #2: Clean up any squeeze-out with a water-dampened rag before it soaks in – especially if it will be stained or clear-coated.

 

Avoid clamping pressure dents. If your joints are correctly made, there shouldn’t be any need for excessive clamping pressure.

Pro Tip #3: Do a trial assembly before applying glue; it’ll show you where joinery corrections are needed.

Sandpaper

Sandpaper is made by bonding abrasive material to a backing.

The 4 types of abrasives used in sandpaper are: garnet (used mostly for hand sanding because it wears so quickly); aluminum oxide (more durable and expensive than garnet; and, the most common abrasive for sanding wood); silicone carbide (used to make 400 and finer grits; also used widely in metal work); and, zirconium (sometimes mixed with silicone carbide; for machine sanding, only).

Backing may be: paper (hand or light machine use); cloth; mesh (enhances dust collection and is very long lasting); and, foam (for highest grits; conforms well to contours). Sandpaper engineered for wood product sanding will usually “open coat”, which provides more space between the abrasive particles to carry away dust.  And to enhance sanding efficiency further, stearates (lubricants, basically) are a built-in additive designed to reduce clogging sandpaper.

Let me be perfectly clear: today’s modern, state-of-art stearate-treated sandpapers won’t affect any finishes, water or solvent based alike.

 

Grit Size. The size of the abrasive particles determines the grit size. The higher the grit number, the smaller the particle. For most projects and when working with solid wood, start with 80 grit.  Step through the grits: 80 – 120 – 180; never jump up more than 50% in grits. And if you’re staining, substitute 150 as your highest grit.

Note that there are two main grit standards: European (which has a P prefix and is more universally accepted; it’s the one I prefer) and American.  Both are the same size up to 220 grit; beyond that European numbers for the same grit are higher, by up to 50%.

Pro Tip #4: When sanding veneer, start at P120 and be careful not to sand through to the substrate – especially around edges.

 

Format. Sandpaper comes in sheets, belts or discs, with the latter two primarily for machine sanding. And they come in a wide variety of sizes.

Hand-held sanding machines

There are a variety of hand-held sanding machines, the most common today being random-orbital sanders. These usually take either a 5” or 6” diameter disc and use a Velcro-style hook and loop system for mating with the sander. Adhesive-backed discs, albeit less common nowadays, are still available.

Belt sanders, which are favoured by pros for working wide, flat surfaces, usually come in 3” wide X21” long belts sizes. 4”X24” is less often seen. And there are pad sanders, which are still commonly used in finishing shops.

Dust Collection

Most of today’s sanders are designed to mate with dust collection systems (shop vac or central dust collectors) and have holes in their platens with corresponding holes in the sanding medium, or for use with mesh backed sandpaper. The better sanders boast up to 95% dust collection efficiency.

Hand-powered Sanding

Hand sanding is best used only when a machine will be too aggressive or distort the edge feature you’re aiming for.

Pro Tip #5: Use flat back-up pads on flat surfaces and profiled ones for – you guessed it – profiles. These direct the sanding pressure most efficiently and help maintain the features you’ve worked so hard to establish.

Prep-sanding Technique

Here’s my tried-and-true prep-sanding method:

 

1. To give yourself a reference for visually checking progress, draw light pencil lines across the entire surface (not including ends or profiles).

 

2. Start with P80. If the surface is unusually rough, I’ll reach for my 6” random orbital sander, otherwise it’ll be my 3”X21” belt sander for the first grit.

Pro Tip #6: When using a belt sander, work the surface diagonally, which lessens chances of digging the belt edges into the wood.  Alternate the direction before moving up to the next grit then do a final pass going with the grain.

 

3. Start along one side, moving from one end to the other, then advance approximately 3cms (1 ½”) along and head for the other end. Repeat this process until you’ve covered the entire surface.  Once all the pencil marks have been sanded off, vacuum off the surface, load the next higher grit size and repeat the process until the surface is ready for finish to be applied.

Pro Tip #7: Still see pencil marks? Go back over the entire area with the same grit to lessen chances of creating dips that’ll happen if you focus sanding on only the remaining pencil mark areas.

Pro Tip #8: Sand end grain – including raised molding sections – to one higher grit size. This provides greater uniformity of finish absorption, especially for stain.  Also, a small pad sander is best for these areas, because its light weight makes it easier to control than other sanders.

 

Want to know more? Stand by for my coming article where I’ll cover staining, sealer and topcoat sanding procedures. In the meantime, let me know if you have any questions or comments. I’m here for you.

martyschlosser51@gmail.com

Woodworking expert Marty Schlosser has been designing, making and finishing furniture for nearly 50 years. Before retiring, Marty served as the president of the Ottawa Woodworkers Association, to which he received their prestigious Danny Proulx Memorial Award. He is also a founding member of Kingston Wood Artisans, the local woodworking club in Kingston, Canada. When you don't see Marty sharing his woodworking knowledge through neatly written articles, you may find him conducting Fuji Spray's annual spray finishing workshops in Toronto, Canada - where people all over North America wait for, every year.

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