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01-17-2010 05:01 PM
What is the finest grit sandpaper you would/should use when sanding between coats of alkyd varnish, and how long would you wait for each coat to dry?
I ask because I am just finishing up my night stands that have several pieces that are a figured veneer. On those parts, I generally waited 24 hours between coats, sanded the first coat with 220, and thereafter with 400. Sometimes I used Abranet and sometimes I used sandpaper, both on a Random Orbit Sander. I didn't keep track of what I used on which pieces . On a few of them, in the right light, I can see "squiggly" scratches that have obviously come from the ROS. So since I didn't take copious notes, I'm trying to figure out what caused the scratches.
I've gotten some great advice and help here in the last few weeks, and as always I know it's difficult to diagnose a problem via forum posts, but I'm just looking for some brain-storming.
01-17-2010 05:23 PM - edited 01-17-2010 05:25 PM
I usually don't go any finer than 320 or 380. I don't use an ROS between coats just a sanding block or pad and light pressure.
If you use polyurthane varnish, 400 and above may give you adhesion problems.
Finishing is an 'Art & a Science'. Actually, it is a process. You must understand the properties and tendencies of the finish you are using. You must know the proper steps and techniques, then you must execute them properly.
01-17-2010 05:24 PM - edited 01-17-2010 05:27 PM
I use a P400 (3M's 216U, sometimes sold as "Sandblaster"), and/or nowoven abrasive pads (3M Scotch-Brite) in maroon, or more often the finer light gray. P400 is about CAMI 320. I do it all by hand, not machine. I am just trying to dull it and remove any defects such as dust and bumps, or the occassional missed hair.
How long do I wait? Until it powders up and does not 'corn' on the sandpaper. This can depend on the temperature, humidity, and air movement.
01-17-2010 07:39 PM
Between coat sanding is done with 320 paper generally if an oil based or waterborne varnish or poly varnish is being used. Sanding should be done by hand sanding in the direction of the grain. Machine sanding should be avoided. The first coat is sanded more aggressively to make the surface flat. Mount your paper on a sanding block. Do not use steel wool or a non-woven sanding pad. The idea is to create and maintain a flat surface.
01-17-2010 08:01 PM
Thanks all. I'm wondering if on figured veneer where ther is no definite grain pattern (pomele sapele in this case), there will always be some amount of scratching or imperfections when viewing the piece in "just the right light". I don't seem to have this problem on the legs and rails where the grain is relatively straight, and any scratching might blend in with the grain.
No one mentioned wet sanding so I assume you are dry sanding?
01-18-2010 12:58 AM
I have done wet sanding but not any more. First, it is too hard to see what has really been done leading to oversanding and cut through DAMHIKT. Also water and wood aren't very compatible, with only partially cured finish and less that the final amount of finish at that to keep them apart and not fighting.
01-18-2010 03:12 AM - edited 01-18-2010 03:14 AM
...two important points. If you are sanding; 1) don't use a grit that is too fine--320P to 380P is plenty, and 2) sand by hand. Your "swirls" are the result of using a ROS. But, I want to offer a somewhat different approach for those applications were the technique can be used. On flat, particularly horizontal surfaces like table tops, don't sand at all. On these surfaces I prefer to scrape the finish between coats. Scraping does a much better job of leveling the finish, it can be done in any direction (with, across, or diagonal to the grain), and it is faster than sanding.
My preferred scrapper is a stiff trapezoid shaped blade of the type use in utility knives. They can be purchased with rounded corners, or you can slightly round the pointed ends with a file before use. Hold the blade between your thumb and forefinger, angle it ever so slightly in the direction of movement, and lightly but firmly scrape the surface. If the varnish is adequately cured you will remove micro thin ribbons of finish. Initially the ribbons with be narrow as you cut away the brush mark ridges. But, after a pass or two the ribbons will be as wide as the blade and will leave a uniformly dull surface indicating that a micro-thin layer of varnish has been removed. Unlike hand sanding, or even sanding with a soft backer block, the hard, flat edge of the blade will span imperfections like the previously mentioned imbedded hair thus not only removing the foreign body; but, also leveling the mound of varnish puddles and builds-up around these invader. Once learned, this scraping technique is much faster than sanding and best of all, it leaves absolutely no scratch pattern in the finish. It is particularly useful when finishing figured and burled surfaces.
Note: I do not use this technique on those occasions when I apply poly. The poorer adhesion properties of poly require the pattern of scratches left by thorough sanding to provide maximum adhesion between coats. A mechanical bond between coats is not necessary with either an alkyd resin or phenolic resin varnish. This technique also works well with water-borne finishes since inter-coat adhesion is accomplished by the slight "burning-in" of one coat to another.
01-18-2010 04:03 AM
I may have this all wrong, but when using alkyd varnish (which is almost always the case) I don't sand between coats unless I'm knocking off dust knibs. The alkyd formulas seem to adhere to each other very well. I do sand to level the finish after, say, 3-4 coats. But that's not always necassary.
01-18-2010 10:19 AM
Thanks Steve, scraping sounds interesting. I'll give it a try on some practice pieces.
Thanks also Fred. I guess I had it wrong - I thought that you HAD to sand between coats when using alkyd varnish to ensure good adherance.
01-18-2010 10:38 AM
As others have said hand sanding, if and when necessary... you didn't say if your ROS is a velcro type but if it is...those swirl marks are from the abranet paper... I had the same thing once and i think what happens is the velcro sticks thru the abranet mesh and swirls the wood