Tuesday, July 27, 2010

Tech Tip Tuesday

Cat recently sent me some pictures of the canoe that she is working on. She's chosen to use White Pine for the strips. It's not a bad choice, really - while not as light as cedar it is fairly lightweight, shapes well and is inexpensive. Some of the downsides include the fact that White Pine can be brittle, isn't really rot resistant and can have pitch pockets. Brittleness isn't a biggie if you can strip the boat with it and the rot resistance should be taken care of by the epoxy and fiberglass encapsulation. The pitch pockets, however...

Let me start with a little story - my father made a pair of nice Adirondack chairs for my DW and I that were out of White Pine. White Pine being a resinous wood, contains pitch pockets which are little areas that may or may not be seen and contain pitch. Being a rather savvy guy, I knew that before I painted them, I should seal them with shellac to keep the pitch from bleeding at any knots or pitch pockets. I then primed and painted over the shellac. The color was a nice hunter green. When I put the chairs out on the deck, they warmed up in the sun and the pitch pockets that I'd carefully sealed bled through. The chairs are still in the cellar - one of these days I'll put them out to see if the pitch has hardened up and I can fix the finish.

Back to Cat's canoe. She says that she has only a few small pitch pockets - about 5 on one side and she hasn't looked at the other. She was debating what to do about them. Here are a few of the pitch pockets:

I'm concerned that leaving the pitch pockets will cause the fiberglass layer to delaminate. As they are small areas, it might not be a huge problem, but it's better to be safe than sorry. One problem here is that there may be more pitch pockets than she can see, but the strips are fairly thin (~1/4") so most of them should be making an appearance.

To deal with this issue, she was thinking of excising the pitch pockets with an X-Acto knife and then filling the void with either a mixture of wood flour and carpenter's glue or a wood putty with mineral spirits.

First of all, the best approach is to avoid the pitch pockets before you put them on the boat. One method is to use a different species of wood that doesn't have them. If you're going to use a wood like White Pine, just be selective about the strips you put on and leave the strips with pitch pockets behind. You can also cut the pitch pockets out and scarf the strips together.

If you've built the boat, carefully excising the pitch pockets with a small sharp knife is not a bad idea. If there is residual pitch, a bit of denatured alcohol to remove the remaining pitch would be OK. Whatever you do, avoid the use of mineral spirits on the hull - residual material absorbed by the hull can prevent the adhesion of the epoxy and fiberglass coating that you'll be putting on the wood. For that reason, the wood putty with mineral spirits is not a good idea as a filler. I had a student try to find glue spots with a cloth dampened with mineral spirits at home. his epoxy/fiberglass layer had issues in the future and he had to remove contaminated areas of fiberglass and put repair patches in.

The use of wood flour and carpenter's glue is not necessarily a bad idea but, if you will be claddin with epoxy there are two issues. First, it doesn't saturate with epoxy and looks "light" in color. Also, epoxy doesn't bond well to the glue. I don't recommend the wood flour and carpenter's glue mix for that reason.

The best ideas is to use Dookie Shmutz. Dookie Shmutz is the name for a mixture of epoxy, wood flour (from the hull sanding) and fumed silica. (Cab-O-Sil is one trade name) The epoxy gives you the hardness, the wood flour the color and the silica is a thickening agent to keep the epoxy from bleeding out of the mixture. Once you've mixed a batch of epoxy, you add wood flour until it is the consistency of warm organic peanut butter. You then add silica until it is a creamy peanut butter consistency. This mixture is applied with a plastic spreader into the checks or voids in the hull. Avoid spreading on excess as it is difficult to sand off and can leave a blotchy finish on the surface of the strips. Masking areas you don't want to coat with tape works well, too - just remember to remove the tape before the epoxy sets.

After you've applied the epoxy filler and let it cure, you need to sand off the excess. Because it is significantly harder than the surrounding wood, if you try to hand-sand this, it will cause the softer wood to sand away first, leaving irregularities. To avoid this, use a sanding block or longboard and work carefully.

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