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Shellac primer is gumming my sandpaper, how to proceed?

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Forum topic by Scorpius posted 492 days ago 1485 views 0 times favorited 8 replies Add to Favorites Watch
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Scorpius

2 posts in 492 days


492 days ago

Topic tags/keywords: shellac sand gumming

Hello all!

I have speakers cabinets made of chipboard and goal is to get black high-gloss finish on them. I know chipboard is awful stuff but these are 20 year old, very famous speakers so I’d like to preserve the cabinets instead of making new ones. They will be painted by professional furniture painters with 2-component paints. I just need to do the preparations. What I’ve done so far:

- sanded chipboard with 180 grit sand paper
- sealed with 2 coats of self made thin shellac (75g shellac chips into 0.5l spirit), 1 coat of self made thick shellac (150g shellac chips into 0.5l spirit)

I did not sand coats between layers because I was in the impression that shellac will self level. Well it did not. I fact, its very rough “orangeskin” like surface. So I filled all the small holes with water based wood filler. Then I tried to drysand it with 240 grit sand paper. → disaster! Shellac gums sand paper in matter of few strokes.

I’m out of ideas now, what is the best way to continue? Some say use 2-component car filler, others say shellac is fine. I wanted to use shellac since its easier to apply but I cannot sand it. Am I doing some mistake with sanding? I do not want to use wetsanding in any case since it could potentially ruin whole cabinets. Parafine oil in sand paper?

I let it dry 1h before new coat and overnight before sanding.

Thank you very much for advices.


8 replies so far

View Howie's profile

Howie

2656 posts in 1508 days


#1 posted 491 days ago

I think you used old shellac if it isn’t drying.

-- Life is good.

View jmos's profile

jmos

681 posts in 954 days


#2 posted 491 days ago

It does sound like old shellac. If you have any left, put a drop on some glass and let it dry; if it stays sticky, it’s old.

I’m not aware of any way to force old shellac to get hard. The ‘good’ news is that shellac is reversible; you can remove the shellac with alcohol. It won’t be fun, but it’s an option.

Good luck

-- John

View Fred Hargis's profile

Fred Hargis

1612 posts in 1078 days


#3 posted 491 days ago

I would suggest something else. Make a pad (this would be a lint free cloth, with a wad of filling in the middle, wool is best). Linen is often suggested for the lint free cloth, but an old tee shirt might work, although it may wear through faster. Tighten the pad to form a ball with no wrinkles on the rubbing side. Load it with DNA, and try rubbing the shellac to be smooth. Having something like a ketchup bottle filled with the DNA is a great way to load the pad, and if you have something to practice on first, you might learn more than working on the actual project. I think you’ll find the shellac (which may be too thick at this point) will level out nicely. I doubt it’s too old if you mixed your own from flakes. Shellac is great for a lot of things, including a final finish in many cases, but it can be tricky to use.

-- I long for the days when Coke was a cola, and a joint was a bad place to be (Merle Haggard)

View Clint Searl's profile

Clint Searl

1368 posts in 946 days


#4 posted 491 days ago

1. Remove as much of the shellac as possible by flooding with straight DNA and wiping.
2. Sand to 220 to smooth the surface.
3. Apply several coats of flat black latex/acrylic wall paint.
4. Top with clear lacquer if spraying, or waterborne poly if brushing.

-- Clint Searl.............We deserve what we tolerate

View Cosmicsniper's profile

Cosmicsniper

2199 posts in 1743 days


#5 posted 491 days ago

You mixed the shellac flakes with spirits? Is that mineral spirits or alcohol spirits? The latter is fine…the former is bad. Use denatured alcohol, since it’d be a shame to waste good whiskey. Heck, I’m not even sure shellac flakes will dissolve in mineral spirits.

When right, it should dry within the hour (it dries in 5 to 10 minutes around here) and be ready to sand. If you did use mineral spirits by mistake, you’ll need to use mineral spirits to remove what you done.

My favorite way to do black-gloss finish is to mix oil-based ebony stain into oil-based poly and spray on. The more coats, the darker the look. Clint’s method works as well, but be sure to keep the paint thin.

To fill the grain, shellac (with DNA) will work just fine. Put it on thick and sand it level.

-- jay, www.allaboutastro.com

View Woodendeavor's profile

Woodendeavor

210 posts in 1191 days


#6 posted 491 days ago

you can use a card scraper to remove what you want. It will not clog

View Scorpius's profile

Scorpius

2 posts in 492 days


#7 posted 491 days ago

Thank you all for advices! I don’t think my shellac is old, since I just made it few day ago from shellac flakes and denaturated alcohol. The shellac layer is thick, but it was intentional; I wanted shellac layer to be primer and filler. By googleing “shellac sanding gumming” I found in other forum that sand paper gums also if you sand shellac layer too fast and press too hard. The melting point of shellac is only 150F so slow strokes and light pressure should be the way to sand shellac layer. Also Mirka -branded “Abranet” special sanding paper is awesome for this purpose because it doesn’t clog so I’ll just buy that.

But what do you pro’s suggest as my next steps? I’ve already filled the uneveness of shellac with water based wood spackle before knowing that method proposed by Fred Hargis could have flatten my uneven shellac layer.

So, to get super flat finish for 2k primer paint should I:
a) sand shellac/spackle to supersmooth with abranet sanding paper (400 grit?)
b) a) + add a layer of thin mix of shellac, then flatten the surface with method proposed by Fred Hargis
c) use method proposed by Fred Hargis to remove water based spackle and flatten existing bumpy shellac
d) some other method.

I really don’t want to ruin my project so all help is very appreciated :)

View mikema's profile

mikema

175 posts in 1171 days


#8 posted 491 days ago

How old are the flakes? The flakes themselves can get too old as well.

-- Mike ---- Visit my woodworking blog: http://sawdustnewbie.com

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