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I have recently acquired a guitar and ended up sanding though the clear coat of it, as described in this question. Now I am trying to find the best way to refinish the body.

In the picture below we can see two different 'zones' that have been sanded (please ignore the bottom left, which has not been sanded

  1. the one on the lower horn: this is where I sanded a lot. I used up to 100 grit paper and I did sand a lot, like removed up to 1.5mm of the top.
  2. The rest of the body where I have sanded enough to remove the finishing (I believe it was tinted polyurethane)

enter image description here

I also (wet)sanded the white-ish area with power-tools. Turns out it was not a good idea, as the sanding paper got quickly blocked with a white-ish material (probably the prepping materials (grain filler, sanding sealer, I guess) used prior to painting). :

enter image description here

We do see differences in texture in this over-sanded area though. At first, I thought that it was because I sanded too much and the pores got more exposed in some areas. However, upon careful inspection of the wood, I now believe that the difference in texture is due to the presence of prepping material in some areas vs. bare wood in others, like shown below:

enter image description here

I now believe that the difference in texture is because one area is bare-wood and the other is (sanded) prepping materials because it looks like the bare wood areas are more sanded and because of the material that build up in the sanding paper upon sanding, as shown below - I don't think that this build-up material can be 'wood', it looks like a synthetic material, like a grain filler or something:

enter image description here

Do you think that the difference in texteture within the over sanded area is due to the presence of prepping material (vs. bare wood)? I came to this conclusion based on the visual inspection of the area as well as the material that builds up in the sanding paper

If this hypothesis is true, do you think it would be possible to 'rescue' the over sanded area with a grain filler, for example? Or would it be best to sand the whole body down to get rid or all prepping materials and reach bare-wood all over the body? Keep in mind that I am talking about a lot of sanding, like up to 1.5mm

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Although your photos are excellent I don't believe it's possible to be sure what's going on here. Without the wood in front of you so you can view it from different angles, try some water penetration tests, even just feel it, there's a lot of missing information. But what I suspect is that the darker colour of the lightly sanded area is due to a certain amount of remaining finish (AKA sanding sealer) still being present within the wood fibres.

Even without the use of formal Sanding Sealer the application of a base layer of clear finish, to act as a sealer/pore filler/sanding sealer, can help ensure an even colour (i.e. prevent blotching) when a tinted finish is going to be applied.

In addition, this remaining finish (if it's not shellac) could well have darkened slightly in the years since the guitar was completed. This occurs with all varnishes and most lacquers; the colour difference being subtle but noticeable over the first few years and only starkly evident after 10-30 years.

But now I would guess the best thing to do, to get a more uniform finish, would be to sand back the whole body to obtain the same appearance of the white-ish area.

I think this is likely to give the most uniform final result, but it's just possible you may be able to selectively finish the over-sanded portion. And obviously this would save a great deal of sanding, other further effort, and overall be a safer option.

Preview the finish colour
To sort of test for both sealer and discolouration of same at once, wet the lighter portion with water, alcohol or spirits. This is a classic trick for getting a sneak peek of the colour you can expect from the first one or two coats of clear finish going onto the wood.

Do the wetting right at the transition line. If the colour is practically identical then other than some light filler work I think you're in good shape to proceed. I'm stressing nearly identical here because if the colour is noticeably different you'd have wanted to feather the transition quite a bit to attempt to get a hard-to-notice transition from one area to the next. But I think this suggests leaning in favour of fully sanding the top.

Still lighter?
If the colour is significantly lighter there's a further component that is likely present, and that is the that the darker wood is actually darker.

Almost all woods change colour in some ways over time due to exposure to light, with lighter woods going darker, often expressed as "a deeper colour" which I think could be fairly said about the portion not sanded heavily.

Unfortunately this result does pretty much demand sanding the rest of the top until the colouring is uniform.

Grain filler?
Conventional grain fillers are really for filling grain and nothing else. They're primarily or entirely for use on coarser-grained woods (e.g. oak, ash, walnut) to provide a uniform flat surface.

However the product you link to doesn't appear to be a grain filler as the term is commonly used today. Instead it seems to be closer in spirit to sanding sealers, although possible more substantial? Anyway, although it could be a fine product it's simply not needed. Why? Because the first coat(s) of finish can — as always — be used to pore-fill or 'seal' wood.

And I was anyway going to suggest that you primarily pore-fill using your clear finish of choice, after tactically filling the areas of missing wood, the crumbled knots etc. with tinted epoxy. Filling this way is a quick and efficient process with shellac and especially lacquer, slower with varnish, but doable with any of the three (or some combination, e.g. shellac under varnish).

Tip: instead of finishing a bit, sanding back a bit, applying more finish, rinse and repeat as is often recommended I would instead suggest you do it essentially in one go — apply enough finish (as many coats as it takes, which could well be 5+ with varnish, 7-10 with shellac) until you're fairly confident the wood is fully flooded with finish. Then apply one more :-) And then cautiously sand back in a single operation. This is overall faster, and less wasteful of finish than the iterative approach.

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  • thanks very much for the answer, very helpful. I now believe that the differences in texture within the over sanded area is due to presence/ absence of prepping materials - please have a look at my edited question. So one area, more rough is where is still have prepping materials and the other area, smoother is bare wood. What do you think?
    – BCArg
    Commented Apr 10 at 8:36

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