Shellac coming off
Welcome! / Forums / General Woodworking Discussions / Finishing / Shellac coming off
- This topic has 6 replies, 4 voices, and was last updated 2 months ago by
Scott M.
-
AuthorPosts
-
24 June 2024 at 8:08 pm #842467
I finished a cherry coffee table top with dewaxed shellac from a can of Bullseye (the common stuff in North America). Whenever I leave something heavy on it for something like an hour or more, the finish seems to come off when I move the thing. This happened with my heel (that’s what coffee tables are for), a bowl of food over a coaster, a board game box, etc. The attached photos show how it looks with light reflecting on it.
This is my finishing procedure:
– Sand wood to 400 grit.
– Add to the shellac about 30% to 50% more ethanol by volume to thin it down. The contents from the can feel too thick.
– Apply 3 layers with an inexpensive hake brush I got from Blick. The hairs are pretty fine, but every once in a while one comes loose.
– Wait overnight.
– Do an ultra light scraper pass to remove some buildup spots.
– Sand to 400 grit.
– Apply 3 more layers, or until shellac is pretty glossy.
– Apply paste was with 0000 steel wool.Has anyone seen this happen before? I assume this is a problem with my application or that this shellac is of low quality (who knows when the can was produced), so I’d love to get some advice on the finishing procedure, which I believe is very similar to what Paul has demonstrated. I’m also considering getting my own shellac flakes and just dissolving it a day before applying.
Attachments:
You must be logged in to view attached files.Seems strange to me, though I am hardly an expert. I only have two thoughts. First, if the shellac were bad, it would not dry very well. If it dries well, I assume it is ok. Second, when I use shellac or any other clear finish over wood, I never sand the wood to 400 grit. In fact, I don’t go beyond 150 or 180. I am pretty sure I got that from Paul. I do not know whether that would make a difference, but it might be that the shellac needs something to grab hold of. After Paul is finished with his plane, he often lightly scuffs the wood to give it (I think the word is) tooth before putting on shellac or other varnish. So maybe that is the problem. Oh, I often do use some 400 grit (very lightly) after my first coat of shellac or other varnish to smooth it out.
24 June 2024 at 8:38 pm #842474Are you sure there is no glue residue on the piece? And did you sand it before or is it white when it lifts. ?
I agree with Sanford as far as sanding. Based on Paul’s instruction (and my own experience following his example) I don’t sand past 240/250, and even then just very lightly, and really I only sand to give the wood some “tooth” so the shellac has something to hold onto. I feel like sanding to 400 prior to shellac wouldn’t give that amount of tooth. I also wonder if you’re applying too thick? You said you apply 3 layers then wait overnight and apply 3 more layers, so are you not allowing it to dry between those layers? Shellac is not like polyurethane … the layers do not build upon each other, instead each coat will dissolve the previous coat and combine with it. If you are applying “3 layers” at once, it will be very thick and won’t be able to dry properly. The result should almost seem like part of the wood, rather than a coating over it.
30 June 2024 at 4:41 pm #842932I go with a 400 grit new sandpaper because I remembered Paul saying to go lightly with a worn 250, but maybe I should try with just a straight up 250 grit based on everyone’s feedback. Thanks.
The sequence of passes I do like in Paul’s videos. He applies one right after the previous has dried (matter of minutes), waits for it to fully cure before sanding (matter of hours), and then applies more. I understand shellac doesn’t build in layers because it dissolves into what’s already there, but don’t know which other word to use in English, so maybe “pass” or “application”? As long as it looks dry (based on light reflection) and the brush isn’t dragging on the previous application, I apply the next one.
Does anyone have a good recommendation of the solution / cut to use for brushing shellac? I don’t know what the Bullseye can is, but it feels too thick for brushing. Maybe I should go even thinner with the solution, so I have to build it up with more passes?
I did a first attempt at french polish with the same can on a tambourine and it’s not having this problem, even when I grip it for hours while playing. I remember sanding to 150 before I sanded to 400, so this could point to issues in one of both of the thickness of each application and the sanding I was doing before for the table.
I see, I misunderstood what you meant about the layers (I think “coat” is the word you’re looking for). I do think 6 coats is quite a lot, although I suppose it depends on the intended use, but I’m pretty sure I’ve never applied more than 4 coats, and usually I only apply 3 coats. I also use the Bullseye (Zinsser) shellac, straight out of the can, applying somewhat sparingly with a hake brush. The couple times that I’ve had a large surface to finish, I did cut it 1:3 with denatured alcohol (ethanol) — 1 part alcohol to 3 parts shellac — just because that made it easier to apply over the large area. If it’s an average-sized coffee table I’d probably cut it to prevent the brsuh from sticking as you go, but that’s not much of an issue for something smaller like an end table.
So my general procedure is:
– lightly sand with 240 grit
– apply the “sanding coat” of shellac (this first coat is absorbed rather quickly, so I’m not too concerned with how thick it is)
– let it dry maybe 30 minutes
– lightly sand with the same (now used) 240 grit
– apply another THIN coat of shellac and let it dry several hours
– then I make a judgement call whether to sand again or proceed directly to the 3rd coat. If my 2nd coat was blotchy/globby I’ll clean up the bad areas with a chisel and/or sand again with 240.
– apply the 3rd coat and let it dry overnight.
– at this point it’s another judgement call between smoothing globs with 240 and/or using 0000 steel wool
– once the surfaces are completely smooth, apply wax with 0000 steel wool
– let the wax set for about an hour, then buff with a cloth or shoe shine brushI can’t say for sure why your finish is coming off, but my suspicion is that sanding to 400 did not allow the first coats of shellac to penetrate the wood, and therefore subsequent coats were not adhering completely, so those 5th and 6th coats are just hanging out waiting for a coffee cup to come along.
-
AuthorPosts
- You must be logged in to reply to this topic.