How smooth is smooth on a spruce top

[lastchair]

I'm totally new to spruce as I have not had any other woodworking experience since Junior High school, and that was with hardwoods. What I wanted to know is how do you get spruce smooth? Seems like when I sand it, I can see the sanding scratches in the soft parts. When I scrape it, I can see small pits/tearouts in the soft parts. And then after sanding, the soft parts are fuzzy. So then I scrape off the fuzz and see the small lumps.

What grit sandpaper should be used? And should I scrape after sanding to get the fuzz off? When would a surface be ready for stain/varnish? I've heard that any unevenness or scratches will darken quickly. What would be the smallest grit paper to work down to? I'm on 150 right now.

thanks

[Bill Snyder]

Keep going. Some of the luthiers suggest 320 is fine enough, others 600. 150 Really is not very fine.
Start with something along the lines of 100 then go to 150, 220, 320, 400 etc...
You don't wont to skip from 150 to 320 because some of the scratches left by the 150 will take quite a while to get out with the 320. The scratches left by 220 will be much easier to get out.
Also use some type of backer with the sandpaper. Your fingers are too soft and will not sand evenly.

[labraid]

I find fingers work well to back sandpaper when used in conjunction with scrapers and other tools. You want your spruce ready for staining and finishing, and my recommendation would be an unburnished, flexible scraper, sharpened on sharpening stones, used in alternation with 220 grit paper. My routine is like this: 220 - scraper - 220 - scraper - (sometimes one more 220 - scraper, then) - finishing spiel. Of course, none of the 220 or scraper routines are material removal, they are only very light surface prep. At least two routines are enough to get rid of any rude marks in the wood, I find.

clarification: tearout with scrapers is almost impossible to reverse without some sort of sandpaper intervention... the scraper will continue to fall into any ruts until it's been sanded away. Violin makers, correct me if I'm wrong, but did not the ancient masters use the ground to actually fill tearouts? They did not want to ruin their plates' tone for the sake of removing visual imperfections.

[Antlurz]

I've often wondered what sort of experence it would be to sand/finish a mandolin with sharkskin for sandpaper.

And if different "grits" can be had?

Ron

[Caleb]

A luthier buddy of mine gave me a great tip on sanding and getting it even. He uses one of those blackboard chalk erasers as a sanding block. It will move with the contours of the body and still sand evenly. Makes sense to me.

[Hans]

Take it down to 320-400. Make sure you raise the grain several times by wiping down the instrument with a not too wet rag.

[buddyellis]

I like 400 with several 'grain wackings' also. I find it's easier to stain with a somewhat finer grit surface. Don't soak the thing, just dampen it. If you see a scratch or sandpaper mark before staining, it WILL show quite visibly once stained. Don't be afraid to let your stain dry overnight and whack those areas that have sanding marks (this is where the 'many light coats of stain' helps, too)

[Stephanie Reiser]

I second Hans's suggestion of "whiskering" the wood, especially maple. It will also improve your spruce progression. I can only add that if you use denatured alchohol to wet the wood, your waiting-to-dry time is dramatically reduced over water. I start/shape with 60 grit, then go grit by grit to 80, 100, 120, 220, 320, 400. At 220 I whisker the wood 3 times. I never use a scraper, though I am not against them. Just never use them.

[PaulD]

I can only add that if you use denatured alchohol to wet the wood, your waiting-to-dry time is dramatically reduced over water.

This is a dumb question, since you say you do it, but does alcohol raise the grain like water does? You don't need much water to raise the grain, but you're right that it takes awhile to dry before you should resume sanding even when it's just dampened. Alcohol would certainly dry much more quickly... I've never used it to raise the grain before.

pd

[sunburst]

If you are using a scraper the "smooth" is different from the "smooth" you get from sandpaper.
It is possible to finish over a scraped surface, violin makers do it often, but the surface, especially on spruce, is not the same as a sanded surface. The grain lines of spruce will show slight ridges on even the best of scraped surfaces while a sanded surface can be almost completely "flat", at least at first. Also, the wood fibers are cut differently so the surface of the wood is different microscopically and will likely take stain differently.

After scraping to refine the shape, I sand with 80 grit, 120, 220, raise the grain, re-sand lightly with 220, and I'm done. I sometimes start with 60 grit if I need to for any reason.
Under a sprayed finish, a thoroughly 220-grit-sanded surface looks just as good as anything sanded with finer grits. I seldom sand spruce or maple beyond 220 grit, and almost never beyond 320. I only use the finer grits of paper for wood surfaces I intend to polish, like ebony bridges and such. I find sanding spruce and maple beyond 220 grit to be a waste of time that only adds dust to the shop and doesn't improve the look of my finished mandolins.