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I'm a newbie to this community and appreciate being able to ask this question:

Is Blackwood suseptable to hairline cracking across the end grain of quartersawn wood ?

I'm building a solid body guitar from 2 x pieces of very dry Blackwood blanks and after routing the body shape, sanded the body edge with 80 grit at the advice of a cabinetmaker, with the view to move up in grit to 320.

After a few runs around the body I noticed some scratches where the 2 quartersawn blanks meet on the bottom end of the body.

So Ive tried sanding them out with 120, but after removing a good couple of mm, I am doubting these are scratches from the 80 grit. They seem to just keep going into the timber.

Is Blackwood susceptible to cracking across the end grain ? The photo shows the area, the hairline cracks runs at 90 degrees to the Blackwood grain.

Thank you, Donenter image description here

  • Yes, these marks are hairline cracks, that look like coarse grit scratches. This might be a bit of my ocd, but I was thinking I could touch up the vertical dark grain bar -like patterns with a matching colour using an artists brush where the cracks cross the verical bars, to take attention away from this 'flaw'. But maybe I just need a Bex and a lie down. – DonnyB Sep 11 '16 at 10:51
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I don't know Tasmanian blackwood at all I'm afraid so can't be specific about that, but all wood has the potential to crack in this way. This doesn't mean the drying was carried out poorly, although an excessive tendency towards this would generally indicate this sometimes it's just inherent to the wood (sometimes just the individual board).

It's not what you asked about but as you included details of your sanding I wanted to comment on a few aspects.

sanded the body edge with 80 grit at the advice of a cabinetmaker

80 grit is not a finishing grit, it should never be used unless you're dimensioning or shaping wood. I presume what you were doing would be classed as a finishing operation and unless something unplanned for has occurred you should never need to resort to 80 when finishing.

100 is about the coarsest you would normally need to use and frequently 120 is the starting grit of choice, coarse enough to do what's needed not too slowly while not leaving excessively obvious scratches.

Obviously the quality of the routed surface is a factor here and routing operations, if carried out properly, should leave a surface good enough that only minimal sanding is required. If not, look at your router speed and feed rate, as well as the sharpness and design of the bit.

You can nearly strive to have the routed surface be finish-ready. A quality router bit that's very sharp is an important starting point for this but doing the lightest of skimming passes as a last step should always improve results. One way of doing this is by running tape around the template, or on the router's fence, when doing the major shaping and then remove it for one final skimming pass (taking off wood equal to the thickness of the tape used).

with the view to move up in grit to 320

Consider sanding to a lower grit on the face grain (220/240) and a higher grit on the end grain (400 or even 600), to help even up the colour once finish is applied. Sand the end grain first, then do the face grain as a final pass sanding by hand using a block.

  • Yes, I was dubious about the 80 grit advice, but, after the amount of 120 sanding I've done since (need to be careful I don't end up with a (ukulele),the scratches, which don't show up that well in the photo, must be cracks. I will definitely try 400 grit and to 600 as you suggest, for the colour evenness. – DonnyB Sep 1 '16 at 10:31
  • opps, hit the wrong button...so to finish.... so thankyou for that tip. SO is there nothing I can do to hide these horizontal marks , which I can still feel with a razorblade edge ? – DonnyB Sep 1 '16 at 10:32
  • @DonnyB. What finish were you going to use? If you'll be applying a full film finish it will fill the cracks eventually. If you'd prefer to fill them first I'd use epoxy glue, mixed with fine sanding dust... you'll have lots of that now! Tape around the cracks before filling so you don't get epoxy soaking into too much surrounding end grain. After the epoxy has set and you've pared & sanded it flush apply your finish. When you're done the cracks should be very hard to see. You'll obsess over this but remember, only you know they're there :-) so try to judge from arm's length not 6" [contd] – Graphus Sep 2 '16 at 7:40
  • @DonnyB. I've done epoxy fills like this on the end grain of knife handles, even knowing where to look they're hard to make out. When you can see them they just look like features of the wood, not cracks any longer (sort of like medullary rays). – Graphus Sep 2 '16 at 7:41
  • Thanks Graphus. I intended, subject to better advice, to lightly stain the back and sides, spray sanding clear over the whole body, and block sand back to level. – DonnyB Sep 2 '16 at 11:08

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