Re: finish on elm?
you can fill the cracks and knots at any stage, ok it will stick better if it is to raw wood but if it's a crack well it might not be an issue even if it is finished, just scrape it out a little. I like to put masking tape around the crack or hole , then use a putty knife and water or oil based wood filler. you can doctor the filler color with powdered pigments or if it is just a little too light after the oil is on, just use a brown felt pen to darken it to match, then apply the topcoat when happy with the color. the tape keeps the putty from covering more area than absolutely necessary to fill the void. I just pull the tape off when it is wet, then when dry flatten it with a card scraper. I am doing a lot of cracks in my floors that way and it's the easiest method I've come up with. I don't see the advantage of using sawdust, just strive to get a good color match and you won't see it. use a couple of shades of pens or putty to make it so the color isn't too uniform throughout the repair and you won't see it. most putty patches show because they are too uniform in color and cover more area than needed.
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