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I learned recently that boiled linseed is not a great finish when exposed to any amount of water. I have a piece near a sink that gets a few drops now and then, and these have become lighter spots due to the water evaporating.

I would do the work of unmounting the piece, a bit of sanding, then another coat of BLO then a couple layers of oil-based satin polyurethane on top. However I do not have time to wait the minimum of a few days for the linseed oil to cure before applying the PU.

Maybe since they are both oil based, I can use them in conjunction, though I am afraid of an even more blotched finish if I blot BLO on the water spots then go forth with PU. Any suggestions on restoring the uniformity of the original finish before immediately applying a couple coats of PU expediently so I can remount the piece and get the area functional again?

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  • Hi, welcome to Woodworking. Out of curiosity what is it you're looking to beef up the finish on? Re. the main thrust of your Q, I'll add a formal Answer later if nobody else has in the meantime but yes you can apply oil-based poly on top of fresh linseed oil without any problem. Done it myself many times. But since you're happy to sand back a bit you might be able to skip the oil and go straight to the varnish. Try this in an inconspicuous area first, but from what I've seen firsthand there's >50% chance this will work (variables being how much BLO was applied before, and the species of wood).
    – Graphus
    Mar 9 at 7:00
  • Wasn't there a reply Comment here earlier??
    – Graphus
    Mar 10 at 12:03
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Yes you can begin to apply oil-based varnish such as polyurethane on top of wood you've applied BLO to. You actually don't need to wait any time at all for the BLO to 'dry', you can begin to apply the varnish immediately without any ill effects1.

I've done this many times myself, but I should note that I am conscientious about removing excess BLO from the surface (as you're supposed to, and as modern instructions tend not to emphasise enough). Results might not be as good if you leave the surface slick with oil prior to the varnish going on; the varnish will still dry and then cure, just expect it to take longer.

However I do not have time to wait the minimum of a few days for the linseed oil to cure before applying the PU.

Bad news for you here, the varnish itself won't be ready for use within a few days. It'll take a few weeks.

Varnishes can't be relied on for waterproofing or scratch resistance until they have fully cured, and typically cure time for oil-based varnishes is given as one month2. The most you can rush this is to wait about a fortnight, but you're taking a risk and do bear in mind this is highly dependent on local conditions as well as the varnish itself of course since all varnishes don't cure at the same rate.

In addition, the finishing process alone will take longer than a few days!

You want to apply a number of coats of varnish to build up good water resistance and generally you apply one coat per day.

If you want to pare this down to the bare minimum you're still looking at at least 16 days:

  • Day one you sand and oil the wood to remove the water marks, then apply the first coat of varnish.
  • Day two hopefully you can apply the second coat of varnish and then comes the hard part.... waiting two weeks.

1 The oil in/on the wood and the oil within the varnish just meld together.

2 In ideal conditions. And note this is one month after the last coat is applied, not the first.

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