January 21, 2009

Sanding Blocks

Always use a sanding block

Sanding blocks are some of the most important tools you can have. Even a scrap of 2 x 4 (sanded flat) is better than just using your hand.

Nearly every project requires me to make a new sanding block or seven for specific applications. More often than not a better fit can be achieved by deliberately cutting a part slightly over size and sanding it to the final shape.

Also see

Home Made Sanding Blocks

Flat Sanding Blocks

I have dozens of sanding blocks if you count the little sanding sticks I make, but only a few of them get frequent use. My most used blocks are cut from 1/2" medium density fiberboard (MDF). Each block is double-faced with the same grade of paper. I have a dozen of three different sizes of these blocks ranging from 50 to 400 grit sandpaper.

Another block I use is made of oak that is 3/4" thick. I sanded it flat on both sides and glued a piece of 1/16" neoprene to one side to soften it slightly. That is the face I use most often. On the other face is a piece of 1/2" black foam rubber that is relatively firm. I use that face for compound curves such as fuselages and wingtips.

A variety of sanding blocks

Top left — An oak block with 1/16" neoprene on one side and 1/2" firm foam rubber on the other.

Top right — Four T-Bar sanders.

Bottom row from left to right

Small dowel wrapped with sandpaper.

Paddle-sander.

Flex-I-Files bow sanders — Excellent for plastic models and hard to reach areas of any model.

1/2" MDF fiber board sanders faced on each side with sandpaper. These are the blocks I use to sand sheet balsa tail surfaces.

Various purpose made sanders for sanding spar notches, etc.

1/2" MDF Sanding Blocks. Each block is double-faced with the same grade of sandpaper held on with spray glue.

These are my general use sanding blocks prepared to get new paper. Each block is made from 1/2" MDF and is double-faced with the same grade of sandpaper.

I clamp the block in a bench vise and use a heat gun and spatula to remove the old paper.

After the paper is removed I use lacquer thinner to clean off residual adhesive. When the thinner is evaporated, I put the new paper on. It's not something I enjoy doing but it's necessary.

In normal use the blocks last about a month. It was even worse when I had two different grits on each block. Eventually I made more blocks so I could double-face them and change the paper less often.

Note: I don't make sanding blocks this way any more. The 1/2" MDF is so inexpensive that it's actually less expensive to throw them away than it is to clean old glue from them. Now I use carpet tape to glue on the paper.

Making Sanding Blocks

I cut my general use blocks from 1/2" MDF. After you cut your blocks, sand them flat. The best way to flatten a block is to spray glue a full sheet of sandpaper to a piece of glass and hone the block on it.

The MDF I use is already pretty flat so it only took a few seconds using 220 grit paper to make them perfect. Hit all the corners with a sanding block to knock off the sharp edge and radius it very slightly. The small radius (maybe 1/16") is important to ensure your blocks don't gouge your work if you don't put it down perfectly flat and also helps prevent the sandpaper from getting caught on a raised edge and tearing from the face of the block (which ruins the block).

Sandpaper comes in 9" x 11" sheets. It is best if you size your blocks to use the paper efficiently. Use a dull razor blade to cut the paper and to trim it after it is glued to the block.

Spray Glue Method

Spray a light, even coat of spray glue on both the block and the paper. Let the glue get tacky (about 30 seconds to a minute). Put the paper on the block and then turn the block so that the paper side is down. Roll the sanding block onto all its edges to ensure the paper isn't curled up at the edge. This is why you radius the edge slightly.

Apply pressure to the block or clamp it down for about 15 minutes. Trim off the excess sandpaper all the way around.

I used a permanent marker on all four edges to indicate the grit on the blocks so I can identify them easily. The lacquer thinner I use to clean glue from the blocks also dissolves the marker unfortunately. You can write directly on the sandpaper using a Sharpie marker and it won't come off but if you stack the blocks you also can't tell which block is which without pulling them all out.

By the way, on the adhesives page, I mentioned that I use cheap spray glue for things like making sanding blocks. That used to be true, but the paper kept coming loose. Loose paper is annoying and can damage the work. I use 3M 77 spray adhesive now. It costs a lot more, but it doesn't come loose, so it's worth it.

Carpet Tape Method

I made a framed tray to lay several blocks in side by side to tape several at the same time. The tape is fairly expensive and it's not the right width for some of my block sizes. If I made blocks individually I would be trimming off a lot of excess tape and throwing it away.

Be sure the block is absolutely free of dust and crud. It should feel smooth and flat. Anything between the block and the sandpaper ruins the sanding block.

Lay down tape over the block and then burnish it down well using a squeegee or something with a firm, straight edge. Cut your sandpaper slightly oversize and lay it upside down on the workbench. Wipe any dust or crud from the back of the paper.

Remove the backing from the tape and attach the block to the sandpaper.

Apply pressure for a minute to ensure a good bond. The best way to do it is lay a blank sanding block on top and use a couple clamps. Hand pressure is ok.

Trim excess paper and tape from the block by guiding an old single-edge razor around it. This ruins the blade so don't use a good knife. I can get about 20 blocks from an old razor. The corner of the blade rounds over but it stays sharp enough to cut the paper cleanly.

Now roll the block on all its edges. Mark the block with the sandpaper grit and it's ready to use.

	A standard sheet of sandpaper is 9" x 11". It is best to size your sanding blocks to use the paper efficiently. I suggest that you divide the sheet into individual sheets for one size sanding block instead of cutting off one piece at a time. Sometimes the pieces you cut won't have the grit marked on them so be sure to write it on the backs as soon as you finish cutting the sheet apart so you know what grit paper it is later.
	I'm currently experimenting with using double-sided carpet tape to apply sandpaper to sanding blocks. The tape is 1-7/8" wide. I made these blocks 1-7/8" x 5-1/4". Because I double face each block with the same grade of paper I can cut eight sheets to make four sanding blocks from one sheet of sandpaper.
	These blocks are 2-3/4" x 5-1/4". I can get six pieces for 3 double-faced blocks from a standard sheet of sandpaper.
	These blocks are 2-1/2" x 8-3/4". I can get two double-faced blocks from a sheet.

Sanding Block Rack

	This rack is very simple to make and helps keep your sanding blocks organized. The reason I made mine is because I was stacking my blocks on top of each other and every time they rubbed together it rubbed me the wrong way. I wouldn't throw my good kitchen knives in a sink to clatter together for the same reason — they need to stay sharp.
	I glued a short stub of wood in my table saw sled and ran a piece of 1/4" light ply through the saw. With each cut I moved the plywood over so that the new groove was over the wood stub. That ensured identical spacing. The edges were routed on my router table.
	The bottom was glued on first using carpenter's glue. It is held perpendicularish to the back by propping it against magnetic fixtures using film canisters filled with lead shot. Vertical presses on the fixtures clamp the bottom firmly while the glue dries.
	The dividers are cut from 1/16" aircraft ply and glued in the grooves using carpenter's glue. When the glue had set up for a little over a day, I applied several coats of Watco Danish Oil using a foam brush. I used paper towels to remove excess after letting it set for about 30 minutes. I applied two coats a day for three days. I'm not real familiar with Danish Oil and it seems to take forever to dry. My test has always been if I can smell the paint then it's still releasing solvents and isn't fully dry yet. In this case that was a couple weeks.
	The completed rack. Now I just need to find a place to put it. If I knew in advance where it was going to go I would have drilled holes to mount it before I glued it together. I may mount it to a wall or on top of a bench. For the time being it's just going to drift around the shop until I have a good place for it. In the foreground are sheets of sandpaper pre-cut to fit the blocks I use. I won't have to cut more paper for several months.

Commercial Sanding Blocks

One reason I do not like commercial sanding blocks is that they almost always use too much sandpaper. Sandpaper is wrapped around the block and clamped in place using a method that never gets the sandpaper taut or flat on the block. The edges of the paper always curl away from the block toward the middle of the block. If you're using a moderately stiff-backing paper this curl can put grooves in your balsa and lite ply.

Because the paper is loose on the block, grit and dust can get between the block and the paper. This dust prevents the paper from laying flat on the block, will cause the paper to wear out faster and makes the block more likely to gouge the wood you are sanding. Sandpaper is not supposed to make the surface worse!

This is why I always glue sandpaper to blocks using spray glue. I only use as much sandpaper as is necessary to cover the face of the block so very little gets wasted.

Another problem with commercial sanding blocks is that they are always the wrong weight. To properly sand something, the sanding block should feel solid, at a weight that provides feedback of what's going on while you are using it. If you use different blocks they'll give you different feedback so you have to learn to understand what each block is telling you when you're using it.

It is hard to describe, but when you are using the right sanding block you know it by feel. You should usually use the largest sanding block you can. Obviously some things will limit the size of the block (such as room for you to hold the part), but a larger block will make it easier to sand things flat.

The only time I will sand without a sanding block is when I have gotten down to final smoothing of a component that has compound curves and I want to knock off the facets left from using a sanding block. I use very fine paper (600-800) that will not drastically change the shape. 400 paper is not very fine when sanding soft balsa and can change the shape of it noticeably in a short time.

One little trick I like is for sanding small, flat parts. Instead of trying to hold the part in my hand, I put it on a sanding block and then use another block to sand it. If you do it right you can get the sanding block the work is resting on to grip the part so it does not move when you sand it with the second block.

You can also put a flat sheet of sandpaper on the workbench and move the part over it. However, you should rotate the part because it is difficult to put even pressure on the part and you may sand it to an unintentional taper.

My other most-used sanding block is a T-bar that is 22" long. That one gets a workout when it is time to sand wing panels. I also glue sandpaper to my T-bars even though sticky-back sandpaper in rolls is available. Sticky back sandpaper is not very good quality to begin with and it also tends to peel up. It also costs much more than regular sheets of paper for a given quantity.

No matter what type of paper you use, you should clean the sander with solvent before putting new paper on. I normally use lacquer thinner to remove leftover glue goo and then acetone to remove any trace of oils contained in the lacquer thinner. If there's just a little glue then I use acetone only.

There are times when you want a sanding block that is firm but flexible. So far I haven't found any material that seemed just right.

I was asked to try Dura-Blocks which are machined firm EVA rubber (which is Ethylene Vinyl Acetate resin and not rubber at all) sanding blocks in several sizes and styles.

I received a 7-piece Dura-Block kit to evaluate. The set includes six sanding blocks and a bar of pumice-type soap that wasn't include in the set provided for review.

The set came in a colorful airbrush art box featuring two creepy clowns. This box isn't doing good things for my psyche because I thought clowns were creepy even before Poltergeist made it normal to think clowns are creepy. I think I'll hide the box under my bed so I never sleep again.

Three of the six blocks are machined to the shape shown below.

One of the blocks is a rod approximately 1-1/4" in diameter.

The remaining two blocks are rectangular cross section.

I haven't had the opportunity to do a proper review yet so this is just an overview and my initial impression.

The machine-shaped blocks are fairly thick and comfortable to hold. While they aren't as rigid as a hardwood sanding block they are fairly firm.

I wouldn't use them for flat sanding balsa wood structures but I think they will be right at home sanding items having long or compound curves such as turtle-backs and balsa cowls.

I think these blocks will really shine when blocking primer and paint. None of the sanding blocks I normally use are water-proof but these are which means I can use them to wet-sand which is the best way to sand a finish.

I'm not real clear on how sandpaper is supposed to be attached to these blocks but my plan is to spray a light coat of spray glue on the sandpaper only. I won't apply glue to the block because I suspect the solvent used to remove the glue will eat the foam. When the glue on the sandpaper is just drier than tacky I'll attach it and see how well it holds. I'll make adjustments to this method as needed.

Purpose Made Sanding Blocks

I frequently make new sanding blocks. I used to quickly throw them together so I could get on with my work, but I've realized that most of these sanding blocks would be useful again, so now I take the time to make them right so that they'll last.

I make sanding blocks to make spar notches a perfect fit. I use a piece of hardwood that is narrower than the slot and make sure it is perfectly square. I glue sandpaper to one side of it and slowly and carefully sand the slot with the ribs clamped together until a spar is a nice fit — not too loose and not too snug.

	I made a set of paddle sanders after using one for several years and realizing how useful it is. These are great for sanding areas where a standard block won't fit. For example, a paddle sander made it a lot easier to sand an airfoil shape on cabanes. Sanding Paddle Pattern (45 Kb)
	This is a blending/contour sander I made for sanding large convex surfaces such as fuselages. I made a large, easily gripped handle for comfort and control. The base is made from a piece of aluminum that is thick enough to remain rigid in use. The sanding face is made from 3/8" thick adhesive backed neoprene foam rubber that I purchase from a hobby shop while stationed in Germany. Sandpaper is wrapped around the side, pulled taut and secured using double-stick tape. Solvent will ruin the foam so I don't glue the sandpaper to the foam although the block would work better that way. This one is too large for most of my work and has been mostly useless. But it's the right idea. The smaller blocks I made in the above linked article get a lot of use and work very well.
	I searched for this material online but all I found were larger suppliers from whom you must request a quote. None of the sites I visited had photos so I'm not sure that the product was the same thing I've used. I couldn't find this material in my searches but I received an e-mail from Tim Terry with a link to a company that sells it at a very reasonable price. Thanks Tim! Foam N’ More & Upholstery, Inc. I received some samples from Foam N' More and it's exactly the right stuff. I used it in the Blending/Contour Sanders Article.
	My thanks to Mervin Friesen for sending photos of a purpose-made sander he created for sanding beveled leading edges. The sander is made from 3/4" particle board. The sander has a 30° angle on one side and a 45° angle on the opposite side. Mervin is currently working on a full-scale Sonex. Take a look at his site here:
	A very simple tool that is accurate and consistent.

Lastly, a great tool for sanding hard to reach areas is primarily marketed to plastic modelers. They are little bows with a piece of plastic sandpaper ribbon that can sand around curved surfaces. Unfortunately, the sandpaper is only about 1/4" wide, so it would take a lot of skill and patience to sand a long, straight surface with them, but I found them great for getting that last little bit in-between cabanes where my smallest blocks wouldn't fit. They are called Flex-I-Files and are available from plastic model supply companies such as Squadron mail order.

If you have a wire-bender, you can easily make your own bow sanders using aluminum rod. Simply make the bow the size you need, cut strips of sandpaper the width you want and glue the ends so they will loop around the bow. You will probably have to do something similar to the Flex-I-Files by threading the ends for a screw so the sandpaper does not slide off or down the bow.

Also see

Permanent (Tungsten Carbide) Sanding Blocks

There have been a variety of tungsten carbide sanders on the market for some time now. Most popular with modelers are the various Permagrit sanders. I have not used them because I do not find the shapes they are available in to be very useful. However, a lot of modelers like them and I can see no reason not to use them other than what I have already mentioned.

Also see

Kentool Sup-R Sander Tungsten Carbide sanding block evaluation

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Model Builder's Guide to Selecting Sandpaper
Painting Tools for Model-Builders

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