Okay, here is my problem:
I have two coats of Watco Danish Oil on my small box(maple). The first coat was kept wet for an hour and then wiped off every 30 minutes until it no longer bled. It then dried for 72 hours. The second coat was kept wet for 20 minutes and then I wet-sanded with 400 grit and wiped dry. There are sanding scratches in the finish after this wet-sanding. I immediately re-applied more oil and let set for 20 minutes and wiped dry without any sanding. Scratches are still there. When I wet- sanded, I used almost no pressure and only sanded the surface for about three minutes, just enough to make sure I covered the surface, not even enough to build up a slurry.
What’s wrong?
Replies
The scratches are in the wood, not the finish. Danish oil does not make a "thick" film finish like varnish or shellac, but soaks in. The oil has highlighted the sanding scratches that are in the wood, probably left over from an earlier, coarser sanding. The only solution is to re-sand and remove the prior sanding marks. 400 is probably finer than you need to go -- 220 ought to do it.
Mike Hennessy
Pittsburgh, PA
tbird,
I agree essentially with Mike's advice, except for this. Not knowing what size sanding grit made the scratches makes advice about which grit to use to get rid of them difficult. Technically speaking, you need to:
1. return to that grit, and sand to a perfectly uniform abrasive pattern, then procede up through your grits, removing the previous sanding pattern with each successively finer grit,
or,
2. return to a grit one "step" higher than the one that left the scratches, sand out the scratches and procede up through your grits to final sanding.
I would use method 1 as it will actually go faster.
Then refinish.
Rich
I agree with Mike and Rich, the scratches are in the wood. The only solution is to resand the piece to eliminate the scratches. Use coarse enough paper to eliminate the visible scratches, and then go through the grits to 220 or with an oil/varnish finish perhaps to 320. You can look for scratches by wiping the piece down with some mineral spirits. Also be sure to look at it both head on, and at a flat angle. Plenty of light is essential.
Then, start over with the Watco. Sanding in the Watco with 400 grit wouldn't create visible scratches--unless you picked up some stray coarse grit from somewhere.
You shouldn't ever let oil or oil/varnish such as Watco dry on the surface without wiping off ALL the excess. Those materials are too soft, even when cured, to make a decent finish.
Edited 2/6/2007 5:27 pm ET by SteveSchoene
Thanks for the input guys. Here is a little more detail, based on your responses. Before applying the first coat of Watco, I had worked up to P320 grit on my ROS (when viewed at low angle into the light, I had a pretty good reflective surface). I vacuumed and tacked the surface and then brushed on the Watco with a foam brush and kept it wet for an hour (as I said before)to allow it to soak in. After wiping off and letting that coat dry for 72 hours, I then applied 2nd coat and kept it wet for 20 minutes and then added a little more Watco to wet sand with 400 grit wet/dry paper. Based on your comments, I agree that the scratches are in the wood, but I don't understand why 400 grit paper would leave scratches in a surface previously sanded with P320. Does this added info provoke any new thoughts?
It does beg the question of why the scratches weren't noticed when you first applied the Watco.
Seems to me that either, 1. you just didn't notice them, or 2. they were caused by stray courser grit, as Steve put it, caught up in your 400.
Any ROS will leave cross grain scratches. For fine work--and an oil/varnish finish calls for it--hand sanding with the grain using the same grit as the last ROS grit is required. You can't have a circular (or eliptical) pattern of scratches in which some of them aren't cross grain. It doesn't take very many of these lining up to make a visible scratch. Sometimes it takes wetting, either with the Watco or with mineral spirits, to reveal scratches.
I would still guess--though can't really prove it--that the scratches were in the wood before the first coat of Watco was applied, and just weren't noticed until the second coat was applied. It's also possible to pick up some stray coarse grit when sanding with the 400 grit. In either event, the solution remains the same, resand and start over, paying particular attention to the sanding and removing all the grit between papers. Sure can be frustrating sometimes.
tbird, Take a small scrap of the same wood you used (if you have any) and sand up through 400 - it shouldn't take much time to do. Make sure it has no obvious scratches, other than those left by 400 grit. (if there are, you have a problem of stray coarse particles contaminating your sanding process) Then, starting with the next coarser grit, create some cross-grain scratch strokes, until you find the one that creates the same kind of scratches you see in the finished piece. You'll quickly be able to determine the grit from which you need to start resanding your finished piece. Rich
Any ROS will leave cross grain scratches. For fine work--and an oil/varnish finish calls for it--hand sanding with the grain using the same grit as the last ROS grit is required. You can't have a circular (or eliptical) pattern of scratches in which some of them aren't cross grain. It doesn't take very many of these lining up to make a visible scratch. Sometimes it takes wetting, either with the Watco or with mineral spirits, to reveal scratches.
Excellent point and one that I completely agree with in principle. However much depends on the grit being used.
A couple or so years ago in the old Knots forum there was a long discussion of grit sizes/visibility of the resulting scratches. It included an amateur woodworker who had decades of expertise as a professional in the abrasives industry. And he maintained that the scratches from 320 grit on up are undetectable by the naked eye. Also included in the discussion was the effects on adhesion between the wood and the finish of sanding with finer grits, with 320 seeming to be the consensus of the finest grit before adhesion begins to be adversely affected. Another guy worked in a shop where they had done extensive adhesion tests and he was the one that offered the 320 grit cutoff point for adhesion.
Taking my cue from that conversation I have since then used 320 for final sanding before finishing (I'm a finisher/painter by trade), exclusively with a pneumatic RO sander, and I never, ever get visible scratches. A lovely side benefit is that the finish builds faster. A dense, hard wood like Maples requires very little scuff sanding after the sealer coat when prep sanded with 320.
I know people love their ROSs but I find that it takes no longer to do a final 220 grit hand sanding with the grain than it would to the 320 grit sanding with the ROS. Plus, there is less noise. I also think I am more "in touch" with the surface and would be more likely to discover defects when hand sanding than when machine sanding. (Being "in touch" doesn't mean to not use a sanding block however.)
I'm with you with respect to sanding a coat of sealer or between topcoats. There I will use my RO to kinda get the worst of it and then go over the entire surface sanding by hand. It's the surest way to get a really nice, smooth finish on a surface which might not be perfectly uniform while not having to resort to a high-build finish to get everything perfectly level. But on raw wood I personally prefer the speed of good quality power equipment.
I am extremely picky about my equipment, though. I only use the very lowest profile sanders. The taller the sander, the more it becomes a foreign object in your hand - the lower or shorter the sander, the more it becomes more of a natural extension of your hand and arm. Which is the primary reason that I loath electric RO sanders. They're just too tall.
I could not agree more. You know exactly where you are when sanding with the grain. Gretchen
Kevin, I concur based on my limited experience with yur observations about ROS sanding with 320 grit. I get no scratch marks and an almost mirror finish on the surface of the wood. When I hand sanded (sanding block) with 320 grit after sanding with ROS @320 grit , I was able to see a lot of fine scratches even though I was sanding with the grain.Thanks.
To all who have posted in this thread, I believe I found the cause of the scratches. Unfortunately it was not thru the exercise of my engineer mind or logic. As I got ready to sand out the scratches, I began to tear off a piece of sandpaper from a full sheet and then realized that the steel ruler I was using to tear the paper with was rusted somewhat. It occurred to me then that I must have contaminated the sandpaper with rust particles that i could not see.So for the future, I will only use a wood straight edge to tear my sandpaper with and will brush the paper off with a good dust brush once I get it on the sanding block.Thanks for all your help. (wouldn't it be neat if one could learn all these lessons once early in your woodworking experience - nah, that would be too easy.)
Wha!!!???
Straightedge!!??
<said with the same tone and inflection as the Tom Hanks character in "A League of their Own," said the line, "Cyring? There's no crying in baseball!!!">
Real men don't tear sandpaper with a straightedge!!!!
Just fold it carefully, then bend it back and forth a few times on the fold to break the paper backing and the front coating. It will then tear perfectly clean on the fold.
No need to thank me!
Rich
That ROS may be part of the problem. I've gotten those "swirl" patterns when I've used discs that had some "gunk" on them. Using a new disc - and a light hand - made them disappear.
Thanks to you all. You have given me good ideas for investigating this. I'll post back to let you know what happens.
tbird2
I'm courious why you'd go to all that hassle to finish something in oil? I mean I grab a can of shellac, over thin it, brush or wipe it on, wait 15 minutes, sand with 220, apply a second coat wait 30 minutes, sand and apply the third coat..
You now have a deep rich (and not platic looking) finish that is a breeze to touch up or go futher with. Two hours and that includes clean up time and a nice glass of wine to celebrate..
In case you aren't aware of it I am the worlds worst painter.. But when I over thin it it flows out smoothly without brush marks. If I miss an area, (I told you I'm the worlds worst painter) the next coat melts perfectly into the first coat and looks great! You can't tell what a poor painter I am..
There are two "tricks" to getting flawless shellac finishes..
First over thin it.. 3 parts of denatured alcohol to one part of shellac.. Don't use paint thinner or anything else. Denatured alcohol!
Second
paint fast!
sloppy doesn't matter. Fast is important! Sloppy fixes itself.. slow causes problems.. Don't worry about runs, drips, or errors.. fast!
Try it, you will have a new wonder finish! I promise you that like others here, you will fall in love with it!
PS If you aren't, simply wipe off the finish with dentured alcohol.. It comes off completely very easily, today, tomorrow, or 100 years from now..
Yep, listen to frenchy.
Having nothing at all to do with the scratch issue - don't use oll/varnish. Use shellac. In 2 days you can have a finish that is far, far superior to an oil/varnish that has taken 2 weeks to apply. It will be a real finish that is far better in clarity and grain/color development and durability than oil/varnish. Shellac is very hard. His "overthinned" method is very good, easier than oil/varnish, especially if you haven't done much shellac finishing.
Rich
Frenchy and Rich14, I am trying a finishing method by John Paquay that he originally posted elsewhere back in '94. (http://home.insightbb.com/~jpaquay)It sounded like it would give a great look and feel even though it would be time/labor intensive. I wish I had run across you guys and your suggestions for shellac before I started this. Thanks. If I do another project like this, I will definitely do the shellac.
I'm going to have to disagree with some. I've done a lot of pieces that were finished with Watco and other "oil" finishes. And one CAN get scratches in the finish after the first coat has hardened. I have done exactly as you have done and gotten the same scratching. The solution is to go to a finer grit. After the primary coat(s) have hardened, 400 grit is too course. Or, better, go to 0000 steel wool. I would now wet sand your work with very at least 600 grit wet paper, then steel wool it. Continue to build up the finish. It is a false conception that Watco does not build..... it will especially if you are letting it set for a period before wiping it down. Shellac finishes are fine but an oil finish gives a different product. Learn the differences so that you can choose the one you want. If you're looking for foolproof, then oil is the one to stick with.
What you're saying makes sense to me. Watco is a relatively soft "finish" and is more prone to clogging up sandpaper than a harder finish, which will powder more easily.
I've noticed that if I get on a harder finish (I use pre-cat and conversion varnish almost exclusively) too soon I get visible scratch marks in the finish that I can still see after shooting another coat of finish over it even if using something as fine as 400 grit sandpaper. The same finish with the same grit of sandpaper later in the cure cycle yields no visible scratch marks. I've noticed the same thing with automotive catalyzed epoxy primer/sealer. Even 600 grits causes visible scratches in the paint IF the sealer was sanded too soon in the cure cycle. And the reason why seems self-evident - the uncured material clogs up the sandpaper and lumps up causing what is effectively (for the too soft material being sanded) a courser abrasive.
So it would make sense that something as soft as Watco could do the same thing and for the same basic reasons. Using a lubricant like Mineral Spirits or Water should minimize the problem. I know it does on my varnishes and automotive sealer.
Sapwood,
You make some very good points and I completely agree with the advice to apply an oil/varnish using steel wool (4-0) as an applicator, abrader for the process. Yes, an oil varnish can be built up to a measureable, but very thin film.
But, while an oil/varnish finish is very easy to apply, it really takes many days (2 weeks or so) to do the job right and it needs to cure for several weeks after the final application to obtain maximum hardness. It is still much less durable than shellac.
There is no question that a well-applied oil/varnish finish can be very beautiful, especially on certain woods, especially tropical hardwoods. But it eventually loses its luster, and frequently appears to completely degrade to look unfinished.
Shellac, with all its advantages of fast application, can be applied to obtain the same close-to-the wood look of an oil/varnish. Once applied, its delicate luster will not deteriorate, and, absent alcohol, ammonia, repeated abraision or persistant, standing water will not need further attention.
But you are right, the OP should experiment with all these techniques, live with the results for a sufficiently long time, and decide what's best for his needs.
Rich
Sapwood, Kevin:Thanks for your info. I really appreciate your info. I'll keep it mind as I go forward on this project.
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