Varnishing River Redgum Slab - white staining appears after sealer coat.

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Forum topic by goodbadugly posted 07-22-2014 02:22 AM 1895 views 0 times favorited 4 replies Add to Favorites Watch
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2 posts in 1373 days

07-22-2014 02:22 AM

Topic tags/keywords: river red gum varnish bondall white splotches sealer coat

Help! I’m building myself a desk from a kiln dried River Redgum slab 1.9m x .9m x 43mm thick. I’ve researched finishing to death, and had decided to go with Bondall monocell Gold Marine Varnish – Satin. The slab was beautifully surfaced when I recieved it and I have only needed to use a finishing orbital sander with 100 grade, and then hand finishing with 100 grade along the grain to create what I thought was beautifully prepared and ready to varnish. I vaccummed the slabs and surrounds (brush nozzel on the vaccum cleaner), and then used a tack cloth (mineral turps) to get rid of all red dust off the surface. I then let it dry. I applied the first sealer coat – 15% thinned with Mineral turps. It all seemed to go well looking wet with no imperfections, until it dried. To my horror, large sections seemed to have a white powdery look, white spots appeared everywhere, and where I had birdseye – open cavities which i wanted to retain, and which were cleaned out thoroughly with bristle and vacuum beforehand, a white powdery look has appeared at inside these cavities and around the edges. I’m hoping these will dissappear if i sand back the sealer for the next coat – instructions are to sand back using 240 sandpaper, but I’m worried if I do and put the next coat on I will be locking in these white splotches and surface marks. Does anyone have an suggestions on what this is, and importantly, is it locked into the final finish , or do i need to sandback completely and start again with some other finish?

4 replies so far

View DonBoston's profile


81 posts in 1430 days

#1 posted 07-22-2014 08:54 PM

I remember once when I tried to thin down spar urethane, that I got similar results. It seemed to me (a rookie in this world) that the spar just couldn’t be thinned down.

Tried two different batches, one at 50% and one at 15% and got the same white marks that you’re describing.

-- Don Boston RECreations by Don

View bondogaposis's profile


4688 posts in 2319 days

#2 posted 07-22-2014 09:30 PM

I don’t know about the white spots, how about posting a picture? One thing is that I would sand to a finer grit than 100. Typically 220 before applying finish. Always test your finishes on piece of scrap especially when trying something new.

-- Bondo Gaposis

View gfadvm's profile


14940 posts in 2658 days

#3 posted 07-23-2014 12:43 AM

Don, I have wiped on LOTS of thinned Spar Urethane (MinWax) and have never had this problem. Did all the woodwork in my daughter’s clinic with wiped on Spar Urethane thinned with MS.

-- " I'll try to be nicer, if you'll try to be smarter" gfadvm

View goodbadugly's profile


2 posts in 1373 days

#4 posted 07-23-2014 07:31 AM

Hey guys thanks for your words of wisdom. I think I’ve sorted the problem. The white splotching is sanding residue (fine sawdust) mixed with mineral turpentine.
After I sanded the slab, I ‘blew’ the fine sawdust out of the birdseye and other cracks by blowing with a straw. But, I didnt get all of it out. I then used a tack cloth to wipe the slab down, but now realise I had the tack cloth too wet with mineral turps, and when it hit the sawdust in the grain and other tiny imperfection it actually made it invisible. When the slab ‘dried’ again, (i.e. the tack cloth wetted down the slab), the timber looked dry but the sawdust was still invisible. So I merrily went off and applied the first sealer coating. When IT dried, the sawdust changed to white!.
Today, after sanding ready for the next coat (with 240 grade this time!), I got a meat skewer and gouged the residue out of every crevice and hole, it was now easy to see as it was white. Where it appeared as a stain, I sanded it off. I’ve just put the second coating down and I am looking forward to (...hoping…) it will be all good when it dries.

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