Knots

[Brad Wood]

Hi

What is the best practice for filling knots to bring some of the cracks and divots level with the rest of the board?
I've got some wood with a fair amount of knots, which is good, LOML wants them.

I don't know the final finish yet, likely some sort of oil then varnish as the top coats (maybe a dewaxed shellac as a seal coat).

I'm thinking some sort of epoxy or something like that, but I want to make sure whatever I use will be clear, sandable, and will accept my finish.

thanks

[Peter Quinn]

I usually use Devcon 5 minute epoxy for shallow knots with deviations and minor surface imperfections I am trying to fill but not hide, West systems for deeper knots or to stabilize things if I suspect they may come loose, and a dutchman (ie remove material with a router, add a patch, plane and sand level before finishing) for things that are just too ugly to look at. Sound knots don't usually require much work, very loose knots may be best avoided, particularly if planing is involved.

[Cliff Rohrabacher]

What is the best practice for filling knots to bring some of the cracks and divots level with the rest of the board?

The best is to buy wood that doesn't have knots.



I've got some wood with a fair amount of knots, which is good, LOML wants them.Shellac over 'em and when they fall out glue 'em in with epoxy.


I don't know the final finish yet, likely some sort of oil then varnish as the top coats (maybe a dewaxed shellac as a seal coat).

That may be best. I have used flat spray lacquer on knots that refused to stop bleeding through the paint and it worked pretty well


I'm thinking some sort of epoxy or something like that, but I want to make sure whatever I use will be clear, sandable, and will accept my finish.Nothing you do will meet those requirements.
Knots move at rates dramatically different from the rest of a board. They are harder so they won't sand the same as the board, leaving high spots with the knot proud. they shatter when being planed and jointed in a machine. They can stop hand planes dead.

About the only machine that treats well with knots is a drum sander. So long that is as the knots don't come apart and rip up the drum and paper.


Knots are knotty. There's a reason they are called knots.

If your missus likes 'em then she's likely also loving the rustic look that knots are part of. ERGO: don't fight 'em Rather make 'em part of the over all work and of your approach. Then when they fall out in a year use epoxy to re mount 'em.

[Robert Parrish]

David Marks showed how to handle knots very effectively in one of his shows. You might check his website to see if you can find it.

[Frank Drew]

Brad,

What everyone else said.

Additionally, oil then shellac then varnish seems a bit labor/time/material intensive, no? Couldn't you get away with only one or two of those (I'd skip the oil if you plan a film finish over top)?

[Brad Wood]

so Peter - once you fill the hole, are you able to apply a varnish over the top of the epoxy?

Cliff says

Quote:

I'm thinking some sort of epoxy or something like that, but I want to make sure whatever I use will be clear, sandable, and will accept my finish.
Nothing you do will meet those requirements.
Seriously? Nothing will accept a finish?

[Mark Kosmowski]

Would CA (superglue) work to hold the thing in place as it is being machined?

For after machining, prior to finishing, would it work to fill the knot space in with a clear acrylic gel medium (http://www.goldenpaints.com/products/medsadds/gels/gels.php)?

I know that oil paints can go over acrylic paints, but don't know how well this will work for wood finishes. I'm working on a table right now with a couple knots and am planning to use Bush Oil to finish it and was thinking to try some acrylic gel medium to fill in the knot prior to pre-finish sanding.

Probably will try this on the bottom first in case something goes south. I figure the knot is expected to take finish differently than the rest of the piece, being cross grain, so it might not be noticed if the acrylic medium impacts the uptake of the finish.

Being an oil-based urethane finish, the Bush Oil might well even just get sucked into the knot from the surface surrounding where the acrylic medium was applied.

This probably adds more questions to the OP's original question, but hopefully will be helpfully thought provoking.

Best of luck with your project!

[Brad Wood]

I've already machined the wood down to final dimensions, all the knots planed fine... had already done this when i did this post.

I'm just looking for something to fill some of the holes prior to finishing. I'm not too concerned with the knots coming out. If they do, well, i guess I'll address that when the day comes. humidity in my house stays mid 40's all year long, so I don't think I'll get a lot of movement.

I'm going with epoxy. Everything I can find says you can varnish over it, so I'm gonna give it a shot.. I plan on doing it on some scrap first just to make sure I get the results I am looking for ;)

[Peter Quinn]

Brad, yes, you can varnish over epoxy. Clear coats might not stick well to very smooth epoxy, but when you fill a knot by building the epoxy a bit above the surface of the wood then sanding it flush, you will not have very smooth epoxy but a patch with a perfect scratch pattern for bonding. I am not guessing at this, I have done it, it works fine.

Ever used Bondo? This is an epoxy with a great deal of filler added, used mostly for auto body work. It holds primer, paint and clear coats just fine.

Another example? I did my entire front porch, 500SF, of Doug Fir, with a Clear Penetrating Epoxy Sealer from Smith (this is not a filler but a thin sealer/primer), then applied a pigmented VARNISH (Sikens, alkyd based deck stain) and when applied before the epoxy is fully cured the epoxy actually keys the varnish to the under coat. No failure, looks perfect after 5 years.

The only problem I could imagine would be if you used a very short oil varnish, like a Rock Hard table top varnish, on knotty wood with epoxy stabilized knots. The wood surrounding the knots might move more than the stabilized knots and cause cracks in the surface over time. Of course this might be a problem with unstabilized knots too. Most regular varnishes and polyurethanes are more flexible and shouldn't be a problem. Adhesion will not be an issue in any event.

Be careful filling deep cracks with CA glue. I have had very poor results with this in the past. Sanding heats it up a bit, and it seems to get gummy no matter how long it cures.

[Paul Ryan]

I like certian pieces to have knots in them, it looks rustic. Some people will not buy wood with knots. I do it on purpose quit often, the wood is cheaper and they look cool. But some pieces don't need the nots so I avoid them then. If the knot goes through to the rear of the wood I will fill them from the rear. If it is a hole I almost always turn the piece over because it will still have nice character on the other side. But I try not to put knots of the edges because the wont hold up. What I usually do is. I do my prelimary sanding and take the left over sanding dust out of the cup on my sander. I mix that with a glue that dries clear, usually gorilla wood glue. To fill holes it takes a couple of applications because it sinks into the hole as it dries. Anyway once it dries you can sand it flat, and stain it. It still shows up darker but knots are suposed to do that. It has worked out well for me in the past, and once filled with glue it is stronger than the wood any how.