Sanding before glassing on these boats.

[Barry Pyeatt]

Just thought I might offer this to those of you that are getting to that stage where you are going to glass the plywood.

Softwood plywood, (Douglas Fir, Pine, etc.) have some pretty dramatic differences in hardness between the summer growth and winter growth. When peeled off the log to get the veneer, the sweeping bands of harder wood become very apparent on and across the surface of the plywood.

When you sand this plywood, you are removing the softer material much quicker than the harder growth rings. The result becomes a very uneven surface that you are going to try to apply glass/resin over. You won't see the uneven surface until you get to a later stage unless you run a long batten over it and look closely at the gaps under the batten.

Much of the after glass fill and sanding work can be cut down considerably and in most cases nearly eliminated if you try to avoid much sanding the plywood over all. Knock off the edges and lightly even out the joints. But try to keep from sanding the surface much.

With hardwood plywoods, it is a little different and they sand down more evenly, but softwoods end up with a hill and hollow surface. No wonder so many fill and sand, fill and sand and sand and sand some more. More coats of epoxy and filler add a lot of weight and really don't add to the structural strength of the surface. The bond of the glass is what makes the surface stronger.

This won't be important if you are not planning on fairing the hull and trying to achieve a high finish. But the uneveness transmits thru the glass and epoxy and requires that you fill and sand to get it fair and even.

Save the sanding for after you glass other than to just get it ready to glass. You will get a better bond with the resin if it isn't sanded so smooth anyway. You want an even (fair) surface but it doesn't have to be really smooth.

Just a bit of my own experience over the years for what it may be worth.

[Frank Hagan]

Good point, Barry! Here's a pic of me trying to fix that same problem:

At the time, I wrote:

During my prep' date=' I sanded the port side first, and over did it. The rotary cut fir plywood sanded away in the soft, lighter spots, but the harder wood in the grain pattern didn't sand away as fast. Looking at the hull at an angle, with a bright light, you could see undulations all across the surface. I used some epoxy fairing putty that the guys at West Marine gave away (it was inventory time, and they had several cans of paint and putty that had been opened and were not salable.) It was a light blue in color, and I spread it using a 12" drywall taping knife to fill all the areas I had sanded away. Here, I'm sanding the fairing putty out. In the end, it looked good, but it was several hours of work that I wouldn't have had to do if I had simply sanded the joints and screw hole filling like I ended up doing on the starboard side.

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I spent way too much time preparing the surface, eliminating small scratches and imperfections ... and didn't realize it until after the glass and epoxy was on. It does cover a lot of imperfections, so don't worry about scratches, etc. unless they are major scratches.

[Barry Pyeatt]

Yes, been there and done that a bunch.

One of the reasons I offer this is that I see a lot of builders really working hard on the hull before they glass to get it all smooth. The more that they sand the worse it becomes.

I love ROS and they are great to use, but if used before the glass goes on, they compound the problem very quickly. The area that they cover at any given time is quite small and concentrated on an even smaller area while spinning in that random orbit. I would love to be able to show just how much the sanding is creating more of a problem than it is solving, but it is really hard to see until the damage is done.

I use a long board to work on the plywood itself. Or the belt sander quickly and then get away. The advantage to this is that it doesn't concentrate the effort on a small area. It spreads it out over a broad area with more of a fairing effect than a concentrated effect. Exactly what we are after.

I make up my own long boards for fairing and sanding. They are not quite as nice as the one Peter Stevenson uses in the video but they are the same principle. A wide, flat, semi-flexible surface that just serves as a backing for a long wide strip of sanding paper.

A 1/4" thick piece of hardwood plywood is ideal for a board about 3 or 4 inches wide by up to about 20 inches long. This will flex slightly and allow you to wrap a cut 24" sanding belt around it and secure it at both ends. Since it is pretty stiff, it won't concentrate the effort on a small area, it spreads it out. A 3/8" thick piece will of course be stiffer yet. Fine for long sweeping hull sections. But too stiff for areas such as the cabin top and sides and smaller areas of the cockpit and decks. The decks are not such an issue since they are generally non-skid. But achieving a fair surface is much easier if you can spread the forces out over a larger area while working on plywood. More like a road grader leveling things than a bobcat trying to make it even. The grader knocks down the high points and spreads the material in the low depressions. The bobcat just makes it more of a washboard effect. Lots more work to get to the same place.

When sanding of course we only want to knock off the high points. Then fill the low depressions and sand them even with the surrounding area. The ROS is too concentrated to be efficient in doing that. But it is great at finish sanding overall once it is fair and the primer, etc. has been applied. It is even better at what it is designed for, and that is final finish sanding.

Belt sanders have only a small plate that is the focus of the sanding effort. So it is on and move quickly or you end up taking way too much off and again developing an uneven surface. The belt sander pad is just the metal surface below the center of the belt. Yes, the entire belt will remove material, but it is concentrated across the radius of the roller at the front, that flat pad in the center and again across the radius of the trailing roller at the rear. Both rollers are higher than the pad in the center. Thus, only the small area of the center is the really effective working surface. Hard to level with that small an area actually working. And they are extremely aggressive, designed to do very rough work, not finish work. Again a long board will perform much better over a broader area.

Hope this will help some who just didn't realize these points. Maybe it will save some frustration and a lot of extra work and expense for anyone interested.

[Guest]

Computer garu-website administrator, with a sander in his hand, working in the cover of darkness. So thats the key to the smoothe running of this forum. :wink:

Since the weekender plans have come to the world, advances have been made in plywoods and access to plywoods that can save you bunches of work and grief in the finish work and the overall enjoyment of the boat. I think it could be said that some added emphasis could be made that folks take a little extra time in seeking out the best possible woods for the project.

Just my take and watching so many struggle with dealing with some of the wide varieties of lumberyard stuff. When a person gets further into the projects, there are many times that the issue of a better finish comes into play, that is never thought about when opening up a new set of plans. This happens across most home builder plan sellers in small boats.