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Dealing With Sap


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I posted this in another thread about some on-guitar woodworking, but I figured 1) I might actually get it answered here, and 2) others might benefit from the answer easier than in a non-guitar thread.

I applied sanding sealer to some highly figured redwood, let dry for +/- 20 hours, sanded with 220 grit, and had the residue/dust wiped clean with mineral spirits. A full day later I find tacky wet lines along some of the grain and along all four edges. I think I have sap.

If that's what it is, how will this effect the finish? My only two options right now are lacquer and poly. I'd rather use poly on it just because it dries to a usable hardness a LOT faster than lacquer. But, if lacquer is better with sap (or whatever else it might be), lacquer it is.

If it isn't sap, what would be your best guess as to what it is, and what to do about it?

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Amazing what happens when a little soap and alcohol get into the grain isn't it?

If it is still there this evening/tomorrow morning and still sticky, a little mineral spirits and one of those ultra fine scrubby pads will get rid of it (don't expect to save the scrubby). Once you get it cleaned up IMHO you have two options - shellac as mentioned (which might limit your topcoat options - but is equivalent to fight fire with fire) or grain filler (flame shield on - but it works!). Scientifically speaking, redwood has an open-celled structure - this makes it great for absorbing finishes but if you happen to get a piece with a little resin/sap in it - that can get displaced by your finish. This is why it's used for decks - it absorbs finishes well. There are also different grades of redwood and I would assume you wouldn't skimp, but at the same time much of the stuff that shows up in shops is a mix of heartwood and sapwood from the tree - even for furniture grade.

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Looking into Quarter's un-explained "use shellac" suggestion, I spent some of the massive ammount of free time I have at work with our friend Google. After about 3 different key-word searches, 7-8 pages deep each, I finally found something useful.

Piecing together the info from a few different sources, it turns out that redwod inotorious for two things: absorbing huge ammounts of finish/sealer, and sap. Regardless of the grade or figuring, redwood has a TON of sap.

As it turns out, redwood sap takes just forever to dry up (die). Even after the wood itself has been seasoned properly, the sap can remain active for a few years. Alcohol is one of the main substances used to render the sap inert.

As it turns out, alcohol is the solvent used in shellac.

The main procedure is to clean the piece with alcohol to remove any surface sap, let it dry, then give it a couple coats of shellac. The extra alcohol will give extra "sap killing" power, and the shellac will seal in whatever is left.

From reading Flexner's book, shellac has a shelf life. Using pre-mixed shellac bought from the box stores is HIGHLY discouraged. However, I'm not sure if I want to make the jump to mixing up my own just yet. I'll have to think about that for a few hours.

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There is one store bought canned stuff that can be used with some effectiveness. Its the Zinsser Bulls Eye Seal Coat, its completely wax free shellac, 2lb cut. I still prefer mixing up my own and trust me its super easy and somewhat gratifying, but if you really don't want to that can is about your only bet if you want shellac in a can. Somehow they manage a 3 year shelf life with the stuff, no idea how, so just check the date before you buy make sure it hasn't been sitting forever. For the first couple coats you might want to try thinning it a bit to really attack that sap problem as you were saying. Just find the alcohol to shellac ratio then measure out the 2lb cut and add appropiate amount of denatured alcohol or the like and thin it. The stuff works just fine and might be a good product to switch to for all your sanding sealer needs as it is just shellac. Though, it does give a slight amber tint, which is another reason I prefer mixing up my own. Anyhow, might wanna check it out. You can usually find it at Lowes or Home Depot. Frank Ford mentions it on his site somewhere if you want a professional opinion on the stuff(frets.com). Best of luck. Jason

Edit:Haha Quarter you beat me this time! The score is now 1 to 1!

Edited by jmrentis
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