Same trick,using black epoxy to fill voids, as the people who work
A little tip, you can thin the mixed black epoxy with denatured
alcohol (About 5% max) to get better penetration without weakening the
I think whether or not you can do that must depend on what brand of epoxy
you're using. I've tried that with Bondo brand marine epoxy (which I really
like) and wound up with a rubbery result that would never cure hard. I may
have used more than 5% though; not sure. I've never tried it with System
Three, which is the other brand I use most often, but it's already sufficiently
thin that I haven't felt the need to cut it any thinner.
And I too have done the black epoxy on mesquite thing; I do live in Texas after
"Our beer goes through thousands of quality Czechs every day."
(From a Shiner Bock billboard I saw in Austin some years ago)
My choices of "Good Epoxy" never included Bondo so can't comment.
System 3 has good stuff, have used thousands of gallons of it.
The alcohol add can be tricky, a little goes a long way.
Strictly for special applications.
Black epoxy works well with several woods, not jut mesquite.
Yeah, with a name like "Bondo" one might conjure up the idea that it's
low-quality automotive body putty crap, but it's really quite good compared to
most locally available junk called "epoxy". This is the stuff I'm talking about:
It has nice nice working qualities and excellent strength (and a nice caramel
color too, which blends very nicely with wood without the need for additional
colorants), and I was planning on buying some more when this batch ran out, but
it looks it's been discontinued...
Since you've already tested alcohol with System Three I might try it sometime
if the need arises. I'm not sure it would work with their T-88 product though;
that stuff is a bit of a different animal.
Yep. You see it a LOT with Mesquite though because it's so hard to find any
decent quantity of wood that isn't full of checks and voids.
Free bad advice available here.
To reply, eat the taco.
Decent epoxy, filled with phenolic microballoons to the consistency of
peanut butter. For paler wood, silica microballoons with a dye. You
can apply this with a popsicle stick, you don't need a mould.
Microballoon filler is useful because it's easy to sand afterwards.
Add Cab-o-sil (fumed silica) to the epoxy to thicken it. Make the mixture
about peanut butter consistency. You can get Cab-o-sil (or Air-o-sil) at
most any place that sells epoxy other than HD, Lowes, etc. Marine stores or
Surfboard shops may be a good bet. If not, internet.
You can use clear packaging tape to help shape the fill. Best to apply it
to another piece of wood, clamp that to what you are filling.
Epoxy doesn't sand real easy. A cabinet scraper works pretty well. As far
as seeing marks, depends on how finely you sand. Just like wood.
IMO, a large epoxy fill isn't going to look good...it will look like a
yellowish chunk of glass. Or dark glass, depending on how deep the fill is.
My own preference is to use wood...chips (as from a planer), coarse dust
(saw dust) and fine dust (sanding dust) mixed with glue to build up a
surface that is flat but shows irregularity because of the mix of particle
size. It may or may not be acceptable "as is"; if not use an artist's brush
with paint - any kind of paint - in varying colors to draw in a pattern and
colors that matches the surrounding wood. Then top coat. Do it right and
you'll never know the knot was there.
BTW, if the knot has a dark edge, cut that away before filling and coloring.
I'd suggest mixing up some epoxy in a cup and adding sawdust from the same
kind of wood until the mixture is pretty stiff - it should stand up in peaks
and droop slowly if at all.
It'll still sag some while it's curing, so all you need to do to keep it in
shape is to make some of what used to called "dams": take a couple pieces of
scrap wood, cover them with a strip of smooth tape, and then wipe a thin
film of white grease on the tape so it'll release after the cure.
Do the knots one at a time. Fill the voids with a spatula, tool it until
it's a bit proud all around. Clamp a piece to the underside face and another
to the vertical edge to keep the stuff from drooping out of the patch.
Finally wipe off the squeezed-out mixture and tool the upper face until it's
just very slightly proud (epoxy doesn't shrink to any appreciable extent).
Work reasonably quickly.
After a while, while it's still just a bit green (not yet cured rock-hard),
pop off the dams, give the edge and the underside face a wipe with a small
bit of paint thinner to remove any trace of the grease, and block-sand as
necessary with a coarse paper like 60 or 80 grit to make it flush. Let it
finish curing, and then go over the faces and the edge with finer grits
until you like the look.
The filler will come out darker than you expect, so do a test on a piece of
scrap to see if it's to your taste. If not, there are other special-purpose
fillers for epoxy which come out white or gray, and then you can take a
brown sharpie and just draw over the patches to try to match the grain, or
stain it to suit yourself.
For something like this, I use West Epoxy. You can get different catalysts
which cure at different speeds. For this I'd use slow catalyst so that I'd
have plenty of time to work the patch before it started to stiffen up.
Hardware store epoxy might not give you that flexibility.
Well I've filled all the knots now, and to my eyes they turned out
great. I pre-finished around the sites to seal the pores, and after a
few hours of drying I made dams out of small strips of wood into which
I rubbed paraffin. The epoxy was too thick to flow into every single
crevasse, but it didn't have to. It just had to hold the biggest
chunks in place. So now that's done, and the dams came off easily,
leaving behind perfectly flat and flush epoxy surfaces. I've got a
couple more dutchmans to do, and then the whole face frame will be
ready for finishing and installation. Yeehaa!
Thanks again, everyone, for all the tips!
- Owen -
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