Sanding a Boat Properly

(or It Takes a Lot of Scratch)

by Sherwood Heggen

One of the most overlooked processes in restoring a boat is that of proper sanding. Take a look at the boats at dock at the next show and look closely. Look at what? Look past the shiny varnish to see how the various parts of the boat fit one to the other. The deck, topsides, and transom should appear as one smooth, flowing surface unless there is a purposeful step or change in direction to the surface. Then look at the effect the sandpaper had on the wood’s surface. Is it full of sanding scratches and swirls? You can tell the effort was there, but the knowledge of how to do it right was still to come.
The perfect looking boat isn’t a result of sanding the varnish well. It comes from proper sanding at each stage of construction or restoration from fairing the bare frames to the last time before the final coat of varnish. You will go through a lot of sandpaper in grits from 36 to 600!

For this article, let’s limit our discussion to what the typical novice restorer could expect when doing a strip, stain, and varnish. In most cases, the wood is in pretty good shape, but the varnish and wood are looking tough because of dock rash, warped planking and loose bungs. After the varnish is off, all of these things can be more easily fixed. Sanding plays a major part in converting a haggard boat into a show boat.

With the varnish and stain off, clean up the wood with a thorough sanding before you restain. Before you start, check to see if the planks are laying flat or tight against the deck and topside frames. To do this, place a thumb on a live seam (where the edges of two planks touch) and with your thumb on the other hand, push against one of the plank edges. If the planks are loose, you will feel movement. Not good! Why? Because loose boards are the sign of fastener and wood problems below. If you ignore the loose planks, no amount of sanding will make the plank edges level with each other. Most likely, the fasteners holding the boards in place have lost their grip, and/or, the frame or batten below is rotten providing no anchor for the screws. Remove the bungs in that area and see if the screw can be turned snug. If the screws strip out, the wood below is likely bad and must be replaced. This will require removing the plank(s) in that area to gain access to battens or frames for replacement or repair. Once solid wood is in place and the planks are drawn tight, you will be able to effectively sand the surface fair.

Dock rash should be fixed at this time also. If there is compressed wood or deep scratches, the wood should be swelled to “pop” out the dents. To do so, apply a wet bandage to the dent or scratch. Make a pad 3 or 4 folds thick from an old turkish towel by cutting it to the size to cover the problem area. Wet the cloth and tape it to the surface with duct tape. Keep it in place for a couple of days. Then, remove it and lay a wet towel on that area and apply a hot clothes iron to the area. This will heat the water absorbed by the wood and cause the cells to expand and remove the dent or compressed fibers. Fill the remaining low areas with mahogany colored Famowood. Fill any holes with the same. Keep an eye open for and fill any blemishes as you progress with sanding. Filling them after you are nearly finished sanding can leave a halo caused by filler around the hole filled because it remains in the grain around the hole.

Check the planks on the hull for fairness by rubbing your hand across them at 90 degrees to the seams. Do they have a wavy and uneven feeling? Over time, wood swells and shrinks, and takes on a different shape than when it came out of the factory. Again, screws can be loose and plugs, or bungs, can also stand proud of the surface and must be sanded flush. Replace any shallow or loose bungs at this time. If these conditions are not corrected, they will be greatly diminish the final appearance of the finish. The better the finish, the smaller the problem that will show up.

Equipment to sand the hull fair is as simple or extravagant as you wish or can afford. The simplest, old stand-by for fairing the hull is a flat, flexible length of three-sixteenth inch thick plywood which is 3 x 16 inches with handles screwed to either end. The alternate is the air file which requires a substantial air compressor to meet the file’s demands for air. Use of a random orbit sander is not recommended. It can leave sanding swirls that you will be trying to eliminate for the whole time you are sanding prior to staining. These swirls are virtually invisible until the stain is applied. Then, they show up like millions of little spiral worms all over. Oh! Don’t forget to use a high quality dust mask and change it as necessary. You will be making major dust for many hours so protect your lungs.

The flat board requires a lot of muscle, sweat, and desire to do the job well. The air file will shorten the time required to get the job done but at considerably greater financial expense. A word of caution about using an air file for extended periods is that the vibrating air file can cause problems to your hands and wrists. Use a thick pair of leather gloves to absorb the vibration to help ward off carpal tunnel problems. Take a break every 10 – 15 minutes to wiggle your hands and fingers to get circulation and muscle movement going again.

To get a smooth, flat surface to the wood, it is important to understand what happens to wood when it is sanded. Wood has varying density even within a small area of a board. Sandpaper is made up of millions of sharp points bonded to paper to scratch away the surface. It will scratch wherever you cause it to scratch. The point being made here is that if the surface is to be sanded smooth and flat, use a flat hard, but flexible board to back the paper. A pad between the board and the paper causes the sandpaper to hog out the soft wood and let the hard wood beside it stand proud. The final varnished surface will show a wavy surface. For rounded surfaces such as rounded covering boards or toe rails, use a small rectangle of corrugated cardboard as a backing to the sandpaper with the corrugations running in line with the sanding motion. The cardboard will take the round shape of the surface but won’t dig out the soft wood.

If the planks are all tight, and the wood is sufficiently thick to tolerate some fairing, start fairing the hull with sandpaper as coarse as necessary to do the job. If the hull is very wavy, 36 – grit would be a good place to start with an air file or a sanding board. As the job progresses, switch to 50, then 60, 80, 100, and finally 120 – grit paper. As you get to the 80 – grit paper, it is best to use the sanding board only which would have a broader surface to fair the surface of the hull and deck.

Sandpaper of any kind has a life span that must be recognized by the user. It makes sanding noises when moved back and forth against wood whether it is sharp or dull. Don’t get lulled into a sanding nirvana thinking you are making progress, when unknowingly the sandpaper has lost its effectiveness. If the wood looks like it is being polished rather than sanded, it is time to reload with new paper. The best paper to use is stearated aluminum oxide which is a non loading paper. It will perform better than any other paper. It comes on adhesive back rolls or sheets that can be attached to the sanding board with a spray adhesive.

Start sanding with coarse sandpaper at a 30 – 40 degree angle to the grain of the wood. This will quickly remove the ridges and humps that exist by cutting across the fibers of the wood. It will also cause sanding scratches, but they will be eliminated as you progress through the finer grades of paper. Reserve the very coarse 36 – grit paper for very uneven surfaces. Switch to finer grits as soon as the high spots are taken down. To chart your sanding progress, make zigzag marks up and down on the surface to be sanded with a lead pencil. Each time you go to a finer grade of paper, or as necessary to fair with the current grit, mark the surface with the pencil and decrease the angle of the sanding direction. At the later stages of sanding with 80 – grit, you should be going pretty much in line with the wood grain and the pencil marks will be wearing off evenly as the surface becomes fair. As you progress through the stages, rub your hand across the planks to feel for any wavy or uneven surface. Your hand will pick up even the smallest waver in the surface fairness that the pencil marks won’t. Your sanding to fair the hull will be done when you feel the perfect fair surface. Then, sand it one more time to be sure.

The biggest mistake novice sanders commit is concentrating their sanding effort on a bad area such as an uneven seam or a large sanding scratch. With all of their effort to eliminate the bad spot in that small area, they create a dip in an otherwise fair surface. If bungs stand proud, sand the whole surface around them, don’t sand just the bung. If a dip in the surface exists, sand a large surrounding area around the dip to the lowest level of the dip. When done poorly, the sides and deck look like a golf fairway full of divots. When done properly, there will be a fair line wherever you look on the hull. It is easy to do well when you understand you are sanding the whole hull, rather than sanding a spot.
A tricky area to sand without creating cross scratches in final stages of sanding is where butt joints exist with grain at different angles. An example of this would be at the covering board by the windshield or bridge decks. To guard against cross scratches, apply a strip of masking tape right at the butt joint to cover what needs protection. Then when one side is adequately sanded, apply the tape to the other side of the joint and sand. Be careful that you don’t create a ridge at the joint line. Here is where a random orbit sander can come in handy to eliminate any ridge that might occur. Use 220 – grit paper on the sander with very light pressure. Then go back and use a hard sanding board to carefully sand in line with the grain with 120 – grit to eliminate any circles or scratches that might have crept in. Use finesse to make the area perfect.

Sanding can be a long, boring, difficult job. Many hours will go into progressing through the grades of paper. Most of the work is done with 80 – grit paper to do the shaping and fairing. When the hull appears and feels fair all over, switch to 100 – grit to bring a smoother finish to the wood. Prior to the final sanding, wet the boat down with water to raise the grain and let it dry thoroughly. While it is wet, the surface will be shiny and will tell of any areas that are not fair. Take the time to circle those areas with a pencil and sand the problem areas away. When the surface is dry, final sand lightly with 120 – grit to achieve a smooth fair surface.

At this point, vacuum the dust off using a brush attachment and then wipe down the hull using paper
towels and naphtha. When finished, the surface is ready for stain.

Sanding the varnish has been covered in a previous Gadgets and Kinks in the Boathouse. Refer to the June 2002 issue for that information.

Sanding a hull well brings about a great deal of satisfaction. It is almost a shame to apply stain and
varnish to such a beautiful sculpture. Take pictures in appreciation for the project. Invite friends over to enjoy it with you. Take yourself and loved ones out to dinner and celebrate the job well done!
If you have questions or comments, or want a certain subject covered in Gadgets and Kinks, give me a call at 715-294-2415 or e-mail me at:

Remember, if there is an object remaining that you can recognize as an old wood boat —
Don’t destroy; restore it!