As a beginner I am still a llttle confused. I am using a wipe-on poly of my own mix. 30% mineral spirits. The work piece is an end table for the living room. It is an older table that I am refinishing. I have read on this forum that you should sand between coats to remove dust nibs etc. While I don’t see a lot of dust nibs there a certainly a few and occassionaly a small thread of lint. If I sand between coats, after a drying time of 6-8 hours but before the finish is “cured”, then It is almost like starting over each time. Even using 600 wet/dry leaves significant scratch marks that don’t entirely get covered up with the next coat.
I have also read on this forum that I should not worry about sanding for dust nibs until the final coat. I have read that this one of the advantages of a wipe-on finish. But if I continue to coat over dust nibs, then at the final coat am I not going to have to sand down a few coats to get the dust nib or lint and thereby require more coats etc.
I have already giving the piece about 4-6 coats (I forget exactly how many) and its looking not too bad but there are dust nibs. Will I be able to get rid of those at the final sanding? My plan is to let the piece cure for a couple of weeks after the final coat and then use a polishing compound to rub it out. Will the polishing compound eliminate the dust nibs. It is an automotive compound. I have used it before on a small box (with a wipe-on poly) with great results. I didn’t have much if any of a dust nib problem with the small box.
Thanks for any help
Frank
Replies
Frank,
I use the "thin" version of Poly all the time. I do like to sand between coats. I used 400 grit paper and I really don't give much force on the paper. I almost just move the paper along the surface lightly. Then wipe up the dust and then apply another coat. I will do this for all of my coats and then on the last coat I like to use the synthetic steelwool. Jeff Jewitt's website http://www.homesteadfinishing.com sells a product called Mirlon. I buy the Maroon (esentially #0000 steelwool) and I do my final rub out with the Maroon colored Mirlon. I have had great success with this approach.
Best of luck!
Dark Magneto
How long do you let it dry between coats before you sand?
Frank
Frank,
I will usually apply one coat a night. I am a "hobby" wood worker, so usually when I get home from work I will sand it down and then reapply. So I guess that would be 24 hours. I am sure you don't have to wait that long, but I have a day job so I have to wait until I get home.
Cheers!Dark Magneto
I use an oil based poly 50% or so thinner. I wipe on 3 or 4 coats. perhaps 1/2 hour apart. After several of these sessions I sand the bumps off with whatever sandpaper I have. Then I wipe on several more coats.
Finishing is probably the most mysterious aspect of woodworking. You can take the same set of products, use them with vastly different techniques, and still get a good result.
About the only thing you can do is consider the properties of your chosen material, and experiment to find a "path" (a collection of techniques) that provide a result that pleases you.
You are correct that, with polyurethane (brushed or wiped), you can recoat without sanding -- as long as you recoat within about 24 hours. After the poly has cured (usually about 3-7 days, depending on the product), you then have to sand. This abrades the surface and allows a mechanical adhesion between the two coats.
You are also correct about sanding the last-applied coat to get rid of dust nibs. But here, I am confused. Normally, abrading the surface with 600 grit will not produce any visible scratches (unless you are using a magnifying glass).
I will typically sand the surface with 220 or 320 (to shear off the nibs), and then rub down with a fine synthetic pad to remove any sanding marks. I do this after each coat, on the assumption that succeeding coats will "bury" the dust nibs, and make them more difficult to remove at the end of the process.
As far as rubbing out with buffing compound, you can (and many do), but poly does not lend itself to this process as well as other products do (notably shellac and lacquer).
As an alternative to sanding, you can remove nibs, lint, runs (which you shouldn't have with wipe-on finish) use a single edge razor blade or utility knife blade to scrape off the defects. Hold it just about at right angles to the surface & lightly scrape the surface. Utility knife blade is actually better than a razor blade for this.
I might suggest that a "problem" here is your initial dilution of your varnish so that you are getting things in your finish coat. The usual wipeon is 50/50--doesn't have to be exact but somewhere in that range. This allows the coat of varnish to dry so quickly that dust nibs and certainly lint won't dry into the finish.
You can re-coat in a couple of hours for the first few coats--no sanding needed. After about 3-4 coats, then a light sanding with 320 grit, hardly any pressure, can be good, wiping with a moist rag of mineral spirits.Then 3 more wipeon coats.
I usually use about 8 coats of 50/50 wipeon. I also do not use poly so adhesion is not a problem.
What do you use instead of poly -- tung oil, BLO, or one of the proprietary wipe-ons???
I think you are probably right about the 50/50 thin out -- but it does depend on the product you start with. Some are thinner/thicker than others, straight out of the can.
It is like making bread dough. How much more water (or flour) do you add to the dough?
Well, enough till it's "right."
Most oil-based varnishes, poly or others, are formulated to meet strict VOC requirements and are really much to goopy to use out of the can without thinning. The instructions say not to thin, but that's so they can stay within the VOC requirements. In reality, if you're brushing you've got to thin 10 - 20%, and if you're wiping on, 50% is about right. I'm sure there are exceptions, but that's been true with everything I've used so far.
There are actually varnishes that do not contain polyurethane--oil based. I don't use poly because I don't like the look it gives to the finish. Pratt & Lambert 38 is excellent. I also use marine spar varnish (non-poly), 50/50 for my refinishing.Gretchen
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