Baritone in Tasmanian Blackwood

Hi - just sharing my fourth ukulele - a blackwood baritone with a sitka spruce top and jarah fretboard and bindings. After initial problems with the side bending, things have progressed fairly well. I have some issues with the top, where I have two problems:

1. There appears to be some subsidence between the bracing, which translates to some unevenness in the top and some light patches over the braces after sanding. Is this usual with sitka?

2. Some gaps between between the grain lines after sanding - as if the thin darker lines have "dropped" in places, leaving odd patches where the surface is ridged.

I haven't had this problem with other woods - is it a sitka spruce thing?

Sounds like a humidity(low) problem, the wood has dried and shrunk?

Softwoods can be prone to uneven sanding if the sandpaper is on a soft base. The softer wood between the hard grain lines can wear away quicker. Similarly the bracing and sides support the top and result in more pressure being felt in those areas and more wood being removed in sanding as a result.

How thick is your top? What's your sanding technique like? I'm assuming that you're talking about finish sanding the assembled instrument?

I think titch is basically correct. A top (especially softwood) will "give" under pressure in a way that makes it really easy to sand unevenly. This can happen on a macro scale (i.e. unintentionally sanding more over the braces where it's stiffer) and a micro scale (sanding away the softer grain lines vs the harder). Sanding a braced/assembled top, using too much pressure, or using a soft pad (or - gasp - your fingers) will make it worse. The way I combat this is to get the top down to 150 grit before bracing, then only doing light finish sanding, with a hard pad, once assembled.

Thanks all. Based on your advice, I think I have made the soundboard too thin (my instructions said 2mm for a baritone, I took it down to 1.75 mm which was the thickness I had previously used for a concert / tenor). I may have have also applied a little too much pressure in sanding in selected locations where the "ridging" has become apparent (eg around the rosette and adjacent to some of the binding) - while I use a sanding block (generally cork) I probably concentrated on these areas to remove localised imperfections.

It still looks good at a meter - I want to get to consistently making instruments that can withstand an inspection at 10 cm!

I want to get to consistently making instruments that can withstand an inspection at 10 cm

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Miguel