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question on sanding oil varnish
https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=33243		 Page 1 of 1
Author:  Steve Davis [ Thu Aug 18, 2011 9:57 am ]
Post subject:  question on sanding oil varnish
In this case epifanes..
when scuffing/flatting between coats can i sand through the current coat to the previous one with no penalty, or will that leave evidence in the finish?
TIA
Author:  violinvic [ Thu Aug 18, 2011 10:24 am ]
Post subject:  Re: question on sanding oil varnish
You wouldn't know if you went through the previous coat. What you you want to be careful about is going all the way down to bare wood. For between coats, I use 400 grit and just scuff it to the point that there are lots tiny shiny spots left. That lets you know that you haven't taken off too much. Pals, Vic.
Author:  CharlieT [ Thu Aug 18, 2011 10:28 am ]
Post subject:  Re: question on sanding oil varnish
violinvic wrote:
You wouldn't know if you went through the previous coat.


Vic - so does this mean you don't get witness lines if you sand through a coat of varnish?
Author:  Barry Daniels [ Thu Aug 18, 2011 10:39 am ]
Post subject:  Re: question on sanding oil varnish
You can see witness lines if you sand through, but you will have to have the surface dry and have the right kind of lighting to see it well. But this won't be a bad thing if you intend to apply more coats because they will hide the witness lines. The place where you don't want to see witness lines is in the final coat.
Author:  CharlieT [ Thu Aug 18, 2011 11:01 am ]
Post subject:  Re: question on sanding oil varnish
Barry Daniels wrote:
You can see witness lines if you sand through, but you will have to have the surface dry and have the right kind of lighting to see it well. But this won't be a bad thing if you intend to apply more coats because they will hide the witness lines. The place where you don't want to see witness lines is in the final coat.


Gotcha - thanks Barry.
Author:  Hupaand [ Thu Aug 18, 2011 11:06 am ]
Post subject:  Re: question on sanding oil varnish
How do you get the final coat level if you don't sand it? That's my trouble.
Author:  ernie [ Thu Aug 18, 2011 11:47 am ]
Post subject:  Re: question on sanding oil varnish
Don/t know if this will help back in the 80/s in vancouver I did a lot of oil varnish finishing/refinishing. What worked for me was several very thin final light coats of oil varnish thinned with turpentine to make it flow smoother with a high quality bristle brush. I let this dry for at least 1 week and used 0000 steel wool, with some wax , or just buff dry depending on what you want it to look like . Practice all the steps on a scrap board first.
Author:  Pat Foster [ Thu Aug 18, 2011 12:00 pm ]
Post subject:  Re: question on sanding oil varnish
Hupaand wrote:
How do you get the final coat level if you don't sand it? That's my trouble.


For the pre-finish prep and pre-final coats of varnish, I do my level sanding with an unpadded block backing the sandpaper to make sure I have a level surface to lay the final coat down onto. After a week or so, I level sand the final coat starting with 320, stopping as soon as the gloss has been knocked off, then go to the equivalent next micro-mesh grit (don't recall what it is at the moment) wet all the way to their finest. Then hand buff and polish. That's when I yearn for a buffer.

Pat
Author:  woody b [ Thu Aug 18, 2011 4:51 pm ]
Post subject:  Re: question on sanding oil varnish
Steve the old addage "practice on scrap" applies here. For the first few coats you want to lightly scuff only. After you're getting some finish to build you can start to actually level sand. Most oil Varnishes flow out really nice. As long as you don't have alot of dust in it it won't take too much to level it. You really don't want to sand through. If you're having to do any excessive sanding to get it level you probably need more coats. I want to be able to sand it perfectly level after the next to last coat. If I can't get it perfectly level then it isn't the next to last coat. The "practice on scrap" part comes by sanding through a few times to get a knack for not doing it.


FWIW I don't use Epifanes, but I have used it. The same basic methods apply for most oil based varnishes.
Author:  violinvic [ Thu Aug 18, 2011 6:48 pm ]
Post subject:  Re: question on sanding oil varnish
What Barry said. I try to be leveled when I put on the last couple finish coats. Then I use micro-mesh starting with 3200 and go up to the desired gloss. I have not noticed witness lines on a finished instrument yet. Pals, Vic.
Author:  Steve Davis [ Thu Aug 18, 2011 9:02 pm ]
Post subject:  Re: question on sanding oil varnish
Woody
what do you mean by sanding through?
to the previous coat
or back to wood

Im at 5 good coats no runs, looking good, and wet flatting with 400
pore filling failures have faded with each coat. I doubt i could go totally flat without invading coat 4..
is that OK?
Thanks to everyone but especially you and Laurent for your generous time and expertise in this particular area [clap] [clap]
Author:  woody b [ Sat Aug 20, 2011 8:12 am ]
Post subject:  Re: question on sanding oil varnish
If you sand to the previous coat you can probably see it, but it will pretty much dissappear with the next coat, or when buffing. If you sand all the way to the wood it's pretty much visable forever. 5 coats probably isn't quite enough, but I suppose it would vary with the way you apply the finish. Is there a way you can measure the finish thickness? I measure at the soundhole before finishing, and a few times during the process. A thin finish is desirable, but...................... I want it to be at least 3 mils (.003) thick for protection. I'm looking for a final thickness of 3 to 5 mils.

My varnish, the way I spray it is a little less than 30% solids. I suspect most oil varnishes, at a consistancy for spraying will be similar. A 3 mil wet coat is pretty wet, and likely to run and sag. A realistic coat is more like 2 to 2 1/2 mils. A 2 1/2 mil wet coat that's 30% solids will be around 3/4 mil when cured. 5 coats, with scuffing, and some level sanding is probably 2 1/2 mils or so. If I do everything right I'll have 6 coats, but alot of times I end up with 7 or 8 because I'm sanding out mistakes. I used to do less coats on the soundboard, but I now do the same number of coats on it as well, since it's the most likely place to get dinged up. I don't think there's a negative impact on tone as long as the final finish is under 5 mils, and the extra protection is often needed.
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