Epoxy as a pore filler

[Philip Secrist]

I usually grain fill the backs and sides of my guitars with regular pore filler, but decided to try using PT-40 epoxy. I have applied three coats so far, trying my best to scrape off the extra and not leave ridges with light level sanding in between using 400 grit sand paper. I still end up with sand throughs and reflective specks that are obviously not leveled, although smooth to the touch. What am I doing wrong?

[Philip Secrist]

Here are a couple of pics. It is a little tough to see, but there are unleveled specks and a couple of sand throughs.

[Barry Daniels]

I wouldn't worry about a few small unlevel areas. After all this is filler and you can get the areas level during subsequent finishing. But the sand throughs need to be dealt with because they can show up as a different color when the finish is applied. I would mix up a small amount of your epoxy and thin it with some alcohol. Pad a thin layer on the sand throughs, let it cure, smooth the area with sandpaper being careful not to sand through, then start applying your finish.

[Philip Secrist]

Thanks Barry. What kind of pad and what grit sandpaper would you use? Would you say thin the epoxy to 50/50 with denatured alcohol?

[Barry Daniels]

Not 50/50, that would mess with the setting. Add just a little until it is less like syrup and more like cream. Fine sandpaper like 220.

[Freeman Keller]

I have been using finishing resin (Zpoxy) as both a pore filler and grain enhancer on almost all of my guitars, particularly ones with wood with character. I apply two thick coats in the usual manner with a plastic card, sand back to the wood surface after each with probably 320, then one more coat highly diluted with DNA (maybe 5 to 1 or more alcohol). I use a foam brush for the dilute coat, it mostly all soaks into the wood and doesn't require much sanding, just lightly scuff the surface. Then vinyl sealer.

I did a thread at a different forum,

https://www.tdpri.com/threads/zpoxy-for ... nt.940522/

[Philip Secrist]

Here is the mahogany neck with a couple Zpoxy coatings that were sanded back to the wood. I have ended up with a number of splotchy areas, as you see and the headstock is darker. Do I need to try to sand this neck totally back to bare wood, which I know is tough to do, especially in the heel and start over? This was my first try with Zpoxy. Any suggestions are greatly appreciated!

[Freeman Keller]

Any suggestions are greatly appreciated!

Did you read thru the link I gave you? Lots of pictures of mahogany with Zpoxy.

[Philip Secrist]

Hi Freeman. Yes, I read your thread and what you show is how I wanted the neck to look. Since I obviously did something wrong in my process, I am asking how to back out and get the neck back to a point where I can follow your process. I tried sanding down to the wood before I asked for help on this thread, but could not get all of the neck to have even color (the Zpoxy must have really soaked into the pores!). I had hoped by applying another coat of Zpoxy the color would even out, so I did. But, as you can see, the neck color is not splochy. So, I am not sure what to do at this point. Do I sand the neck until I get all of the Zpoxy out the of the pores and then start over using your process? Thanks!

[Freeman Keller]

Have you tried wiping the neck with either DNA or naphtha right now? That pretty much shows the colors that you will have under finish. You might be surprised that it evens out, which is kind of why I do the highly diluted last applications. What I am looking for is to hold that same color that I get when its wet with DNA.

If you aren't happy with that it might be necessary to sand back. Before I put any finish, stain or pore filler on any wood I always wipe with DNA or naphtha. Remember too that end grain will absorb more than flat grain, but my experience with Zpoxy is that doesn't look that bad. The pics in the link are about the best I can do.

ps - thats a great looking guitar - I like the woods and binding a lot

[Philip Secrist]

I will try that Freeman. Thanks for the suggestions and the complement. Your guitars look amazing! I quit building for a year with all of the pandemic stuff going, but am just getting back to it. Thanks again!

[Marshall Dixon]

Several times I've experimented by mixing the slow set epoxy sold by LMI and 2# cut dewaxed blond shellac 50/50 by volume. Spread it out on a strip of 1/16" fir. It dried hard enough to sand. I thought of using this as a filler coat but haven't tried it yet.

Just used Z-poxy for the first time as a pore filler. The second application gave adequate pore filling but several sand through spots happened. After the 3rd attempt left me with a number of small sand through spots I padded them with the blond shellac and the match was good. On walnut though.

[David King]

One thing I'll point out about epoxy filler which hasn't been mentioned in the thread is that it has a propensity to swell up out of pores during final buffing on high speed buffers (600 RPM +) and then shrink back down into the pores and pulling the finish down with it. The effect is identical to the shrinkage one sees with traditional pore fillers after a month or two. This is why no one in the finishing industry uses epoxy filler, they all use polyester which is much more immune to heat. Epoxy is fine if you don't let the surface get warm during buffing. Either low speed or wet or both is your best bet there.

[Marshall Dixon]

One thing I'll point out about epoxy filler which hasn't been mentioned in the thread is that it has a propensity to swell up out of pores during final buffing on high speed buffers (600 RPM +) and then shrink back down into the pores and pulling the finish down with it...

Thanks for that information. I've been at a loss to figure out why my surfaces are level before buffing and show the indentations afterword. 3 guitars I'm doing now with epoxy fill and had the same problem with all.

[Alan Carruth]

I have used epoxy in the past, and a number of other things, but have gone over recently shellac/pumice filling. Whatever fill material I've used my objective has always been to sand back to the wood, leaving filler only in the pores. This avoids the 'splotchy' look you get from layers of surface coating with different refractive indexes. This doesn't apply, of course, if you're using the epoxy as an under coat to wet the surface for a water based finish, which would look cloudy or veiled directly on the wood.

Epoxy can be thinned out with alcohol or acetone. It doesn't take much to make it pretty runny. That might be useful as a wipe-on to cover any small sand through. Thinned epoxy will shrink over time after it sets up, and with alcohol this can take a while, so it's probably a bad idea for the major part of the filler.

Epoxies are allergenic, and can cause contact dermatitis even in folks who are not actively allergic. Sanding epoxy can be dangerous if you've ever had a reaction to it; it goes from 'sensitive' to 'full blown' in a hurry sometimes, and people have died on the next exposure after showing an itch or rash. It's good stuff to avoid if you can. That's one reason I don't use it as a filler any more.

[Philip Secrist]

Hi Alan, I would love to learn more about how you fill the pores with shellac/pumice. Is the process similar to the first stages of French polishing?

[Alan Carruth]

Yes; basically it's the fill stage of FP. I put on a very thin spit coat of shellac, and switch to putting alcohol only on the pad. Sprinkle a dusting of pumice on the pad and allow it to 'clear' (take up alcohol), and then rub it into the wood surface, using a circular motion. The pumice cuts into the wood, producing dust that bulks up the filler a bit and adds the right color. The shellac sticks it all together and you pack it into the pores with the pad. When it gets to be filled level you allow it to sit overnight of longer, and it looks like you hardly did a thing, but the pores have been partly filled. You keep going, working quickly to the shellac in the pores doesn't pick up too much alcohol and swell up again. After three or four sessions, usually, the surface will be pretty well filled. I let that sit for a day or three and sand back to the wood, removing as little as possible so as not to open up more pores, but getting all of the mud off the surface, where it would look cloudy or 'veiled'. Then I put on three thin brushed coats of varnish, sand back again, and go to building up something like 7-10 coats of varnish for the final finish.

The biggest problem I've had with this is caused by starting out with too much shellac on the surface. You get more mud than you need, and there's no place for it to go, so it ends up on the surface. Because of the pumice it's hard to sand it back, and removing it with alcohol, or even just moving it to another spot, risks removing the filler from the pores, and also causes the filler that's in place to swell. It's better overall to start with barely any shellac, and add a tiny amount as needed. You'll know you need more when the surface feels 'gritty' and dry.

I've been using what's touted to be an 'azeotropic' mix of one part alcohol and two parts acetone for my solvent. It evaporates much faster than alcohol alone. Even adding the two parts of acetone to a normal 2# cut of shellac, which makes it water thin, it still builds up faster because you don't pick up the dried shellac with he pad as much. You do need good ventilation...

All of this goes even better if you oil the wood surface first with some linseed or walnut oil. Wipe it on fairly heavy, and remove as much as you can with dry rags; then go to to the spit coat of shellac. Most of the oil rises to the surface as you work and you take it off with the pad. At least some of the remainder seems to combine with the shellac chemically, and the rest stays in the wood. The coating has a refractive index that's closer to that of wood, so the light penetrates better. This is the old furniture makers way of doing French polish, and I've used this on fiddles. It's probably not a good idea on guitars, since the oil adds damping, which we generally try to avoid. It might not be too much of a problem on the B&S, which normally have lower damping than the top anyway, and affect the sound less. It's hard to say just how much of an effect damping has; it's a variable that's impossible to isolate, and difficult even to measure with much confidence. Still and all, with the limited horsepower we've got, it seems like a good idea to avoid losses when possible.

Fiddles don't suffer as much from lack of power, and there's some evidence that the old boys used something like a FP fill, including the oil in some cases. Maple and spruce don't require pore filling, of course, but when using colored varnish you do have to seal the end grain, particularly on curly maple, so avoid dark spots (think of the early wood in plywood that has been stained).

[Alan Carruth]

I neglected to say:

It does take some effort and time to fill like this, but less than you might think once you get the hang of it. You spend more time waiting for each session to dry out than actually working on it. I always found epoxy hard to sand back, and had lots of problems with pin holes that took time to deal with. I suspect that, in the end, the FP fill doesn't take much more work time, if any. And even with acetone in the shellac I think it's probably a lot less hazardous to your health. Acetone is a normal by-product of fat metabolism, so you have enzymes to deal with it, at least in small quantities. The hardeners in epoxies mimic proteins, which is why they're so sensitizing and allergenic.

[Matthew Lau]

Hey Al,

You're a gem. Thanks for sharing this