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Last year in our renovation we had our builders convert this 19th century mahogany veneer sideboard into a vanity for our bathroom. Unfortunately they didn’t treat the top of the vanity to protect it, and now it’s starting to show some water damage.

I’d like to rectify this and protect the piece from further damage. Based on some initial research I plan to:

  1. Lightly sand the top of the vanity with my Bosch 100w electric sander and 180 grit sandpaper
  2. Apply mahogany stain with a cloth (Rustins wood dye maybe?)
  3. Paint with 3 coats of yacht varnish (Ronseal?)

I’ve pieced this together based on the scant internet research I can find for this particular use case. Does it seem right? Are there any steps in my plan that seem wrong or will risk permanent damage to this lovely piece? I’m worried about sanding all the way through the veneer but am hoping that with a light touch I can avoid this?enter image description here

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  • The one comment I have is that you do need to be very careful using any power sander on veneer. if you don't have much experience using one, I'd seriously consider using an arm powered sanding block. Are you taking the sinks out to do all this? It would make the end product easier to accomplish and look better...
    – bowlturner
    Nov 28, 2023 at 13:17
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    Welcome to WSE. You could also test for the type of finish it is. Test the finish in a hidden spot with a bit of alcohol. If it dissolves the finish, then it is a shellac finish (unlikely). If a bit of lacquer thinner dissolves it, then it is lacquer. Otherwise, you have an oil-based finish (not so easily dissolved). For shellac or lacquer, you may be able to clean it up with the solvents and then apply additional finish. I'm not so sure how well either of those will accept a new varnish finish so that will affect how thorough a sanding you require.
    – Ashlar
    Nov 28, 2023 at 20:39

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In general outline there's nothing wrong with your plan but you can (arguably should) change the specifics.

Lightly sand the top of the vanity with my Bosch 100w electric sander and 180 grit sandpaper

If one wants to lightly sand any surface it's best to start with hand sanding.

Power sanding, particularly if you're not very familiar with your sander and its characteristics or quirks, can easily go too far1. This is especially critical when you're dealing with modern veneers (thin to very thin) but still always a concern when sanding an antique because you can't be sure how much, if any, sanding has previously been done during restoration, refinishing or "upcyling", or any combination thereof.

Here, I presume you're only looking to scuff the surface to 'key' it for the application of the new varnish and for this you really want to be careful to sand just enough, and that's a job for careful hand sanding (or rubbing down with other abrasives). Back the abrasive with a hard block for most or all of the work, rather than just your fingers.

Note: the water rings between the two sinks may present an additional challenge. It is possible that they will require the complete removal of the existing finish. If so I recommend stripping or scraping and highly recommend you don't attempt it by sanding,

Apply mahogany stain with a cloth (Rustins wood dye maybe?)

That may do exactly what you want but be sure to test the colour on an inconspicuous spot before committing to the whole top.

The last thing you want is to apply it liberally, wipe the excess away and then wait out the drying time only to discover that when you apply the first coat of finish the re-stained areas are too dark, too red, or what have you.

Masking
There's a possibility the wood stain may stain the silicone caulk. I think the only way you'll avoid this possibility entirely (other than my final suggestion below) is by masking it off very conscientiously. If you do this you might as well do it before starting the entire process, since the masking will protect against inadvertent damage during sanding.

Remember to wipe away all excess of whatever stain you eventually go with; the on-tin instructions may not emphasise this enough.

Paint with 3 coats of yacht varnish (Ronseal?)

This could work fine, but yacht varnish is intended for (and should be specifically formulated for) exterior use. Obviously this includes lots of water exposure, but it's also to deal with the wide swings in temperature and everyday exposure to sunlight (well at least as much as we get in the British Isles), neither of which apply here.

Interior polyurethane varnish is tougher, more resistant to other damage like scuffs, scratches and wear/abrasion, and is extremely waterproof2 when applied thoroughly and at sufficient thickness. I'd recommend for assurance of a perfect result that you apply it as wiping varnish. A number of previous Answers give more info on where and why you might want to use wiping varnish and how to make your own, but start here.

An alternative to oil-based poly, and especially since you've mentioned Rustin's products throughout, would be their famed Plastic Coating. I've read feedback from pros who have used it in a commercial setting (e.g. on countertops, bistro tables) and even there it has held up over many years — extrapolate from this to home use as "I'll never need to refinish this top" :-)

Note: any varnish will only provide its full waterproofing potential after sufficient time is given for it to cure. Unless your bathroom is unusually warm (and consistently so) take it as a given that you must treat the refinished wood with kid gloves for at least two weeks after the application of the last coat. (I would personally suggest not using it at all for at bare minimum two days after the final coat.)

One last suggestion
You're going to absolutely hate this, but the ideal way to approach this project is to remove the sinks, do what's needed to the wood and then reinstall the sinks.

This allows you to:

  • work unobstructed over the whole surface, esp. handy if you have to go back to bare wood;
  • guarantees you won't damage or stain the silicone;
  • produces a new finish layer that extends underneath the caulk (as it should ideally) to give greater peace of mind for the future;
  • if desired, allows you to spray on a finish instead of brushing or wiping it on.

On the downside:

  • it will cost more;
  • you will be without the sinks for longer, factoring in getting a plumber or builder in twice for the first and last stages.

1 Even when one is intimately familiar with a sander it's still all too easy to over-sand because there's no advance warning — once you see some telltale sign you've already sanded too much.

2 The gold standard — drips don't have to be wiped up but can be left to evaporate with zero water penetration. However, remembering that this isn't Formica or melamine but finished wood, I highly recommend for longevity that you do wipe up water around the sinks fairly consistently; you don't have to be fanatical about it but being fairly consistent about it could extend the varnish life by a factor of two at least (to well over 10 years).

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  • My only comment is that the plumber work here is fairly simple and could be done by the home owner. depending on their comfort level...
    – bowlturner
    Nov 29, 2023 at 14:20
  • Thank you @Graphus, this is exactly what I was looking for - extremely helpful. One follow-up question: when you say that the water marks may need to be scraped or stripped off, would you recommend stripping just this spot and sanding the rest? Will this result in an inconsistent finish?
    – Lindsay
    Nov 30, 2023 at 14:11
  • Re. the rings, sorry it wasn't clear but when I said complete removal I did mean going back to bare wood on the whole top.
    – Graphus
    Dec 1, 2023 at 8:34
  • BTW the rings and the finish losses do make it clear they dropped the ball on that aspect of the conversion, which otherwise appears to have been done well. They either did nothing to the top and were expecting the existing finish to hold up (which is daft, regardless of whether they thought/assumed it was in original condition or had previously been restored) or they refinished in something A, unsuitable for this application (not even suitable for a normal table TBH!) and/or B, applied far too little of it.
    – Graphus
    Dec 1, 2023 at 8:41

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