Black stains on wood from Bessey parallel clamps (not pipe clamps)?

[Eddie Wilson]

Never had this before (least not that I’ve noticed) but I just took some walnut panels out of bessey clamps and had black stain marks where the glue line met the clamp. I had assumed this was only a problem for black pipe clamps. Have I just been lucky up till this point? Was it the walnut?

Ive cleaned my pipe clamps in the past with white vinegar, and then waxed them with Johnson’s wax to easily remove the glue. Other then that just regular parallel clamps.

[robert wiggins]

I've found walnut to rate up there with the best of woods for the black problem. Why IDK, but I do know a narrow strip of parchment paper under each clamp avoids clamp cleaning.

[Matt Day]

Guessing you mean you see black staining where the metal bar touches the glue line? That’s normal. I put some wax paper across the glue lines, but then I have was paper dried in glue to remove. Better than staining though. It’s some kind of oxidation by the way.
Here’s a way around it from googling “Black stain glue parallel clamp”:
https://youtu.be/2eD8O-CpIAQ

[Mark Wooden]

Steel meets moisture(glue) and high tannic acid (walnut) = black rust spots. Always a PIA, Get some canning wax at the grocery store, Gulf Wax is the one I buy. Rub it on the bar of your clamps before you do your glue up and it'll stop 98% of your iron stains. Cheap and easy, doesn't affect your finish either as long as it hasn't been melted in, so don't leave a glue up in strong sun.

[Zack Deluca]

Mix a solution of oxalic acid and water and apply very carefully to black stains, they will come out.

[Richard Coers]

Mix a solution of oxalic acid and water and apply very carefully to black stains, they will come out.
Don't try this without doing some samples. Get a drop or too much on it and you get a spot where the pigment is bleached out of the walnut. I'd have reservations that it could be done carefully enough. Don't you also have to neutralize the bleach as well?

[Eddie Wilson]

Thanks for the confirmations. I guess I’ve just gotten lucky in the past, never seen it this bad or deep. Luckily it’s on the “bottom” of the panels :)

[Glen Crews]

If you don't have PVC pieces handy, a long piece of packaging tape works when laid lengthwise along the black pipe. buying a butane torch (https://eatrbox.com/best-butane-torch/) or buying natural baby wipes (https://toodrie.com/best-natural-baby-wipes-reviews/)

[Prashun Patel]

Those stains may sand or plane out easily if they are fresh; they may not run very deep yet

[Jim Andrew]

I have gone to keeping some 1/4" x about 1 1/2" pieces of scrap on my bench, and dropping on the ends of a glue up where the clamps go, to keep clamp bars from touching the panels. Works for the top only, but I use cauls when I glue up panels, and don't need clamps underneath.

[Zack Deluca]

Don't try this without doing some samples. Get a drop or too much on it and you get a spot where the pigment is bleached out of the walnut. I'd have reservations that it could be done carefully enough. Don't you also have to neutralize the bleach as well?

It can be very easy to spot repair these mistakes using a fine artist's brush and a careful hand. Even one drop of this type of solution is enough to leach the black stain out of the material. I have done this on multiple occasions with success.

Bob Flexner's advice on this matter:

https://www.popularwoodworking.com/article/remove-watermarks/

Good luck.