Best way to deal with sap before finishing?

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Forum topic by AnnaD posted 04-04-2017 03:51 PM 494 views 0 times favorited 3 replies Add to Favorites Watch
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1 post in 874 days

04-04-2017 03:51 PM

Topic tags/keywords: sap polyurethane larch finishing question

So I’m still a beginner in my wood working journey, and have used mostly pine boards from the hardware store or old pallet wood for all my projects, until now. I wanted to make a sleek, modern computer desk for my husband, and wanted one big slab of wood. I found a local man selling Larchwood. I picked a piece that I really liked the color/characteristics (it’s mostly heartwood, quite possible the very central piece of the tree, as it was the widest board he had…and he had basically the whole tree for sale out of his cellar). I noted some sap (or pitch, I’m not sure what the difference is), but assumed I could just wipe it off and the rest would come off in a final sanding. Well! Now that I’ve done my homework about using softwoods, I’m a little worried. I should note both these spots still weep when I give them a firm press. The other side of the board has some minor splits running the length of the board with dried sap inside, right down the middle, just like the cuts pictured
I messaged the dealer and he said the wood was air dried then kiln-dried for 2 weeks. He said I could bring this board back and exchange if I needed to. So, here’s my questions: 1) Is this going to be a long term problem, and was the wood not properly, or only partially dried? It’s a little less than 4cm thick. 2) Should I be worrying about warping and twist in this wood over time? It’s going to stay indoors, but there’s no central air, or air conditioning here in Germany, so the windows are always open in the summertime and there are often humidity and temperature shifts.
3) Am I over thinking this? Will some mineral spirits and my finish of choice (I’m thinking polyurethane) solve my problem and I can go about attaching the legs to this? I could exchange the piece, but it was the widest board and I liked it the best, so I’d like to keep it if it’s reasonable to do so. Actually, my husband wants the table to be wider, so I was planning to go buy a second board if it’s a reasonable endeavor to use this for the table.
4)This is going to be a simple office desk. Computer work, some writing/drawing, maybe a cup of coffee in the morning. I want to preserve the coloring as much as possible. The last project I used resin to finish, and the wood absorbed so much it really affected the coloring, and didn’t give the shiny/polished finish I’m hoping for here. Perhaps polyurethane? Will oil or water based make a big difference for my purpose?

Any insight is helpful! Like I said, I live in Germany and do not have the language skills to seek out help from experienced locals. Every trip to the hardware store is a linguistic journey :) Thanks in advance!

3 replies so far

View Madmark2's profile


476 posts in 1041 days

#1 posted 04-04-2017 07:44 PM

Better wood = better results.


View Fred Hargis's profile

Fred Hargis

5646 posts in 2946 days

#2 posted 04-04-2017 08:40 PM

I can’t answer your question exactly, but I know this (about the sap): MS and then a varnish isn’t going to solve it. Shellac will seal sap, but it almost sounds to me like the wood isn’t quite dry enough (not sure how long it was air dried, but 2 weeks in a kiln is a fairly short time). Does he have a moisture meter to check it, or maybe you know someone who has one? Anyway, for the sap….you cold try a coat of shellac and then see if they still weep through, it might just work. But for the warp/twist it comes down to how dry, and even then if it’s dry enough t may still warp/twist. That’s fairly thick wood, and it would have had to be air drying for quite a while.

-- Our village hasn't lost it's idiot, he was elected to congress.

View Kelly's profile


2397 posts in 3397 days

#3 posted 04-05-2017 01:44 AM

The kiln thing didn’t take. If it had, you’d be able to sand the pitch laden areas and they would powder. If you can merely press on it and get pitch, I have to wonder about the whole “kiln” dried thing. At best, if it was kiln dried, the kiln operator didn’t do his job.

All this makes the piece suspect. Do you have a moisture meter you can test with? This isn’t really that thick a piece, so a meter should give a good indication of moisture content. I’ve worked with six inch pieces for tables back in the day.

If I was stuck with the piece, I’d scrub it with paint thinner, which will dissolve the pitch/sap, let that evaporate off, then seal the areas with poly. Then I’d get on with finishing, being sure to give as much attention to the bottom as the top.

If using a poly finish, I’d thin the first several applications about 25% to 30%. Let the wood suck it up. This will go a LONG way to slowing down gain and loss of moisture that would cause problems.

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