Tuesday, December 13, 2011

Tech Tip Tuesday

I'm going to bring up cabinet scrapers again.   I've covered their care and feeding in this earlier post.  They are really a wonderful tool that many woodworkers and boat-builders ignore at their own peril -   particularly for anyone who does any kind of work with epoxy or other hard finishes.

My students who are building the Ranger had some issues when they were rolling on their epoxy to build the coating up to "kill the weave" of the fiberglass cloth.  I'm not sure if they pressed too hard with the roller or kept the epoxy in the cup for too long and it was starting to "kick".  At any rate, the end result was a surface with a great deal of "orange peel".  If you're not familiar with this, it is a texture on a finished surface that has the look of the outside of an orange - the fruit - with all of the requisite divots and contours that go with it.

I'll say this - I'm not a fond fan of letting my students sand on the outside of the hull with power sanders.  It raises an immense amount of dust in the shop, the sandpaper clogs relatively quickly because they tend to exert too much pressure on the sander to speed the process up, and they have a tendency to cut into the cloth without noticing.   None of these things are good.  Usually, the finish is good enough that a little hand-sanding will suffice to prepare the surface for spar varnish or spar urethane, depending on the type of epoxy used.

With the amount of texture on the Ranger's surface, this wasn't going to take a little bit of hand sanding.  It was going to take a lot.

Students tend to be a bit reluctant to try the cabinet scrapers as they're unfamiliar things to them.  They've been taught about them and watched me use them, but rarely use them on their own.  I find that the cabinet scraper does an excellent job of removing runs from the epoxy and leveling badly textured areas quickly.  It is nice because it makes curls of epoxy rather than dust which are a bit easier to clean up than the dust and don't settle like fall-out on every level surface in the shop.  It is also fairly fast and doesn't clog up the same way that sandpaper does.  It is a lot quieter than a power sander on a large, hollow hull, which makes for more pleasant working conditions, too.

I have a method to deal with this reluctance.

I borrowed it from Mark Twain.  (Think Tom Sawyer and white-washing the fence.)

Let me explain the method.

First, I give the students some sandpaper and tell them what needs to happen to prepare the surface of the canoe for varnish.  I let them sand for about 10-15 minutes which is much more tedious than it sounds.

While they are sanding, I sharpen up a few cabinet scrapers per the instructions in the previous post noted above.  I then take one of the scrapers and proceed to do a relatively quick job of preparing a large area of the canoe with the scraper.  This usually coincides with the point in time when they are getting fed-up with sanding.

When they see what an "easy" job it is with a cabinet scraper, they ask if I have some more scrapers they could borrow and, oh yeah, could I give them a quick refresher course on how to sharpen the scrapers.

Works every time.


Cat Sittingstill said...

Also it's much easier to work on green epoxy than cured--but I'd rather have that green epoxy in curls that fall to the floor than in dust I might breathe.

Though I do hook up the shop vac to the sander and I will say that makes a pretty big difference in the dust factor.

Canoez said...

It is easier, but you don't want to be working green epoxy - for health reasons. Epoxy doesn't finish curing for upwards of a week and the residual chemicals (typically amines) are what cause people to become sensitized to epoxy exposure. It's better to wait and work on fully cured epoxy.

Also, if you try to rush things and apply other finishes (i.e. varnish, paint, etc.) to the epoxy before it is fully cured, they may not properly cure.