In an otherwise attractive piece of wood, an unsightly split or crack can run deep like family turmoil. Most wood fillers are intended to be little more than aesthetic solutions. Even the hardiest fillers are intended to withstand compressive forces but not sheer stress. Here I will demonstrate my solution for when I don't want to give-up on a piece of wood. This block was salvaged from a eucalyptus tree that fell on my grandmother's house (don't worry, I'm fine). I want to be able to work on it without risk of further splittage.
I do all my wood repair at TechShop.
Step 1: Supplies
I have found polyester resin to the be ideal for this kind of repair. It is very strong in it's cured state, it seeps into fibers and produces a strong bond (it's intended use is laminating wood and resining fiberglass) and it is slightly flexible, so it will move with wood as it bends and expands with humidity.
*Not pictured: the necessary plastic bag
Step 2: Prep
I prefer jet black for most wood replair work. I have given-up trying to match wood colors when disguising a crack. In dark wood a black fill won't stand-out much, and in light wood it can look like the figuring of white ebony or cappuccino gelato. It's my piece, don't judge me. Specialized pigments are available for resin, though I have found almost any black coloring works. Here I am using black airbrush paint. A little bit of it goes a long way.
Tape-up the sides and bottom of the piece to keep the resin inside. Cracks run deep, so it is sometimes wise to tape the entire piece inside a plastic bag. Do the repair work before making any cuts or planing on the wood and don't worry about the resin puddling on the outside.
Step 3: Pour
Pour it in the cracks until it overflows.
Step 4: And Use the Bag
Just to keep everything within the confines of the wood, I taped a bag around it.
Step 5: The Unveiling
You got resin everywhere. It's a good thing we used the plastic bag.
Step 6: Finished
Polyester resin can be worked with most woodworking tools. Be careful if you choose to go at it with a chisel, the excess resin will throw-off glass-like shards when impacted. A bandsaw will cut easily and safely, then the resin can be planed, routed, or sanded. This is a structural repair, so the block is now fit for any purpose as a solid piece.