Filling the Cracks

Rev. August, 2003

The hull is done - every last strip has been mounted.  You have scraped off the dried glue squeeze out, and maybe knocked down the high joints between strips, using either scrapers (preferred) or a sharp, well adjusted, block plane.  Now the cracks and dings begin to show.  Before the fiberglass can be put on, those defects must be attended to.

Where do defects come from?

Defects requiring filling arise from several often ignored causes.  If you recognize these causes and take steps to eliminate them, the need for filling is significantly reduced, or possibly (but not probably) eliminated.  I only know of one hull that required absolutely no filling what so ever, and it wasn't one of mine.  It took the builder eleven years of professional boat building to achieve the perfect hull, but he did it.  If you have ever seen a Wood Song canoe (see  you will see what I mean.

Perhaps the most common fault requiring filling is chipped and cracked edges on the fragile coves.  Defects arise as early as the routing of strip edges, when tear out can take chunks out of the cove edges.  Eliminate this one by doing something that is preached against in Basic Woodworking 101 - back routing or climb cutting.  Back routing with a hand held router is extremely hard to control, with the router suddenly wandering off line, ruining the work.  With a router table, back routing can grab the work piece and propel it with rifle-like force.  However - when properly set up and carefully executed, back routing is quite safe and will produce nearly perfect bead and cove strips, regardless of changes in grain orientation.  I have back routed cedar strips with some really wild grain around a knot, and got perfect cuts.  Conventional feed would have blown it to pieces.  

This is the router table setup I use for back routing.  Note that the feed is from left to right, with the bit rotating counter clockwise.  The feather boards must be adjusted to  hold the strip firmly enough so that it has to be pushed through.  If the feather boards are not tight enough, the rotating bit can grab the strip and propel it forward with potentially dangerous force.  See the Making Strips section for details on cutting and milling strips.

So now you have cut perfect edges on the strips - now what?  Excessive or rough handling of finished strips can cause breaks and chips in the fragile cove edges.  Since the coves are cut after the beads are cut, handling the strip after it comes off the router must be done with extreme care.  I like to refer to this as making believe the cove edges are red hot razors, and handle the strips accordingly.  My strips are kept together just as they came off the plank, through the planing and routing steps, to the storage stack.  When I take a strip from the stack, I know that the next one came from right next to it in the plank.  (This only works for full length strips - with scarfed strips, selection for color or scarf joint placement is more important.)  Store the strips away from the boat, stacked so that this order is maintained.  Then you don't have to fumble through a pile of loose strips looking for a good color match, damaging cove edges in the process.

Laying up the strips on the hull can also damage cove edges.  I like to strip with the bead edges out in order to be able to push the strip down for a well seated glue joint.  Placing the strips on the hull can easily damage the coves.  The best way is to have a helper, but this is a rare luxury.  I use a lot of temporary spring clamps to help position the glued strip so it doesn't spring off the hull before it is properly clamped.  Once in rough position, the next step is to immediately push it into its proper and final position and place the clamps and bungees to hold it there. 

If the strips are not fully mated and held firmly until the glue dries, gaps will develop.  The cure is to make sure the strip is fully seated, well glued with some squeeze out, and clamped at around 6 inch intervals along its entire length.  With bungees, clamping pressure can be applied virtually anywhere along the strip.  Areas where the strip is particularly obstinate can have bungee wraps closer than 6 inches, perhaps even supplemented with additional creative clamping.  The stem area is where this can be a problem, since you may be dealing with twist in addition to bending.

The final common problem requiring filling is dents and dings.  You drop a tool on the hull, bump into it while carrying a plank - any number of unforeseen accidents can put a defect in the hull.  Nothing more can be said other than "Be careful".

In spite of all due care, there will likely be a few cracks and gaps to be filled.

Doing the filling

As for filling cracks, there are some things you should not do.  First, don't try to mix sawdust and wood glue and use it for filling.  The color remains that of the glue, and does not match anything.  I tried that once, and wound up with yellow streaks where the glue mixture was.  Second, don't depend on epoxy alone to fill the cracks.  A crack may be all the way through.   If you are lucky and it doesn't run out the other side, you will have a somewhat transparent gap that you will see light through.  Another consideration is the crack has air in it.  When you lay the fiberglass and wet it out, some of the epoxy will run into the crack, displacing the air.  The air then gets trapped under the cloth and a nasty bubble results.  Sometimes it can be pressed out with a squeegee if you catch it before the epoxy kicks off.  If not, the solution is to cut it out with a utility knife and fill with partially cured epoxy.  This is also tricky, since if you cut into the wood with the knife, you will get a dark line where the knife did the cutting.  You can also try to partially cut it only enough to lift a flap, and try slobbering epoxy under the flap with a squeegee.

I prefer using thickened and colored epoxy for filling, since it is also structural.  It will glue strips together at the fill, and will bond nicely with the fiberglass wet out coat.  For filling, the perfect situation is to mix colloidal silica (Cab-O-Sil) with epoxy, and add enough wood flour (extremely fine sawdust) to get the color you want.  Unfortunately, you don't know what the final color of the wood will be where the crack is until you wet out the fiberglass.  A good approximation can be had by taking a scrap piece of strip and wetting it with paint thinner.  The resultant color approximates what the final color will ultimately be.  Mix the wood flour into the epoxy/silica mixture until you match it.  Even this can be tricky, but don't try for perfection.  Be aware that a fill that is darker than the wood will be less noticeable than a light colored fill on darker wood.

If your sander has dust collection, that is a good place to make wood flour.  I don't like to use collected dust after general sanding, since it has glue dust in it and is a mixture of every color of wood I use on the boat.  When I built the mahogany Wee Lassie II, I cleaned out the dust bag on my belt sander, and sanded a bunch of scrap pieces with the sander flat on the bench just to get the dust.  I emptied the bag into a lidded jar, and had plenty to fill the cracks in the boat.  If you are using different colored woods, this technique might be worth a try.

Another caution when using epoxy mixtures for filling - regardless of what you use as a thixotropic agent, it is still a mixture.  The thickener is simply suspended in the epoxy, meaning that on a micro scale, the epoxy in between the thickener particles is still quite low in viscosity.  When you fill a crack, some of the filler gets on the adjacent wood.  The wet epoxy will find its way into the pores and ultimately harden.  When you finally wet out fiberglass over the fill, the surrounding area will show a light colored stain.   The pores have been blocked by the wet epoxy in the filler, preventing the same degree of soak in as the rest of the wood.  The solution is quite simple.  When you fill the crack, scrape off as much of the over run epoxy as possible.  This minimizes the amount of epoxy available to soak in where you don't want it.  Once the fill has hardened, scrape the area to remove every last bit of stain, followed by sanding the entire area.  I like to do my filling after initial fairing, but before the final sanding.  The scraped stain is then long gone by the time I get to glassing. 


                              1                                                 2                                                  3                          

1. - Filled with thickened epoxy, most of the excess removed when filled.

2. - Pro-Prep scraper removes the stain.

3. - Sanded, no stain remains, ready for fiberglassing.

For applying the filler, I like a tool I made from a 3/4" wide cake decorating spatula.  These things have nice wooden handles and are made of flexible stainless steel.  I ground off the round tip and sanded the edge so it was smooth and rounded, much like a very dull knife.  Be careful you don't grind it too fast, or it will overheat and lose the temper.  For scraping, I like the Pro-Prep scraper.  This tool is terrible right out of the bubble pack, but if it is sharpened and honed with about a 45 bevel and then burnished with a hook, it will scrape up shavings thinner than a block plane, even on cedar.  I also use rectangular cabinet scrapers, burnished on both edges.

One last point on filling cracks - It has been recommended to fill large cracks by gluing in small slivers cut from strips.  I suppose that would work, but fitting such small pieces to get a nearly invisible fill is nearly impossible.  I tried this on my latest boat, and didn't like the result.  Instead of a single crack, I had smaller cracks to fill in the fill.  It didn't look as good as a single fill.

There have been other filling media used for cracks.  Susan Van Leuven uses an oil based commercially available wood filler, and Vaclav at  uses a water based mix.  There are pictures of his work where he filled staple holes with it, and it looks quite respectable.  Of course, color matching is key.  Check out his web site, the work shop page.