Highest grit sanding under lacquer

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Highest grit sanding under lacquer

Postby Chuck Raudonis » Sun Jul 08, 2012 11:32 am

I am getting into the finishing phase of my project and I thought I had a plan until I read something in another thread.

The body is bubinga, maple and walnut. My plan is to pore fill with shellac and then sand back to bare wood leaving the shellac just in the pores of the bubinga and walnut. I would think the maple shouldn't retain anything and sand right back to wood. After that come brushing lacquer (don't have a spray booth) with the requisite wet sanding using a progression of micro-mesh all the way down to 12,000 to get the mirror gloss.

Where my question comes in is the sanding of the shellac back down to wood. My plan was to sand down using 400 grit to prep for the lacquer. I just read a post in another thread that said:

"I usually sand to 400 or even 600 grit before the oil goes on, that may be too smooth for lacquer to stick, but it works fine for TruOil or Rockhard Tabletop varnish."

Now I'm concerned that I'm going too far with the 400 grit under the lacquer. Should I stop at 220? Your thoughts?

Here's how the body is constructed.
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Re: Highest grit sanding under lacquer

Postby Mark Swanson » Sun Jul 08, 2012 12:44 pm

400 is a good grit to sand the shellac sealer coats. Shellac by itself isn't that great of a filler of the pores but it'll help some. You can expect that grit to sand through the shellac in spots, depending on how thick you put it on and how much you sand. Much of the shellac will soak into the wood for a few coats.
I don't agree with the statement you quoted. You can put lacquer over as smooth of a surface as you want, because it "burns in". That means it will melt the surface of the basecoat and bind with it. If you have a clean shellac surface, the lacquer will surely stick, in fact melt into it and it'll be fine. Because of that same reason, there is no reason to sand any finer than 400.
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Re: Highest grit sanding under lacquer

Postby Chuck Raudonis » Sun Jul 08, 2012 3:03 pm

My intent is to sand the shellac down to bare wood and just leave the low spots (the pores) filled with shellac. If I do that, there won't be any shellac there for the "burn in" to lock on to. Thus the question about is 400 too smooth.
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Re: Highest grit sanding under lacquer

Postby Greg Robinson » Sun Jul 08, 2012 3:30 pm

Hi Chuck, I think Mark might be on the same page with me here, but I believe that a good quality finish starts with a chemical bond, rather than a mechanical one. The idea that a finish will not stick to a highly polished surface I think comes from people who have left too long between prep sanding and laying on finish, which allows the surface to oxidize, and reduces "surface energy". If the finish is layed on within 10-15 minutes of sanding (ideally), I think it will stick to any sanded surface. It is widely accepted around here that the best glue joints are fresh, and are as smooth as possible (scraped preferred). I believe the same principles apply to finishes.
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Re: Highest grit sanding under lacquer

Postby Mark Swanson » Sun Jul 08, 2012 6:19 pm

Chuck, you're worrying too much and overthinking it. First, it will be almost impossible to sand the shellac and stop right at the point where it's just in the pores. It's easy to open up new pores if you are down to the wood, which will happen in some spots while in others there'll be patches of shellac. It would be better to leave a thickness of shellac to use as a basecoat and not sand down to the wood. And I can tell you that even if you do that you'll still see pores after the lacquer goes on. The lacquer will burn in, and shrink down into the wood and the pores as it cures. Shellac makes a great sealer but a poor filler of the pores all by itself.
If you sand with 400 down to the wood, it will not be too smooth. The lacquer will soak into the wood as it dries.
All you have to do is not worry about this, it's a non-issue. Just go ahead and lay down a good shellac coat - and some just start with the lacquer alone on the bare wood, which is ok too- and then go with the final coats of lacquer. It'll be fine.
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Re: Highest grit sanding under lacquer

Postby Chuck Raudonis » Sun Jul 08, 2012 6:53 pm

Sweet! Thanks guys! That's what I had planned on doing, but that post just got me a bit worried. I'd hate to put all of the work that I did into that body and then have the lacquer peel off in sheets.

I appreciate having you guys around to bounce ideas off of. This is my first build but from the way it has gotten under my skin I think this won't be the last. I'm already looking around and thinking "You know, I could really use an LP and an archtop would be nice for the collection..." God help me I think I'm hooked...

I really like building furniture, but this is so much more enjoyable to pick up and play what you built. I did a test assembly a little while back just to make sure everything was lining up while the body was still "In the square" before I invested the time in shaping the body. I wanted to check the action, make sure the strings were lining up on the neck, make sure the pickups were working, etc. I couldn't put it down for a couple of hours. I just thumped on it even though is was unbelievably uncomfortable to play due to the sharp edges and the fact that I didn't have strap buttons on it. I had a blast. Even more excited because it was truly mine from the ground up. Here is a pic of the test assembly.

Thanks again. I'm sure I'll be bugging you again.
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Re: Highest grit sanding under lacquer

Postby Steve Senseney » Tue Jul 10, 2012 10:52 am

One small comment to make--about the shellac (as I don't do lacquer, except rattle can lacquer on little projects)

You need to be careful that you don't bleed colors from the dark surfaces to the light colored maple when you are putting the shellac on. The way to do this with hand application is by using a clean pad and wiping across the surface ONCE. Then let this dry and repeating the process. After several applications, it will be sealed, and no further bleeding will occur.

As always, practice on scrap.
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Re: Highest grit sanding under lacquer

Postby Rodger Knox » Tue Jul 10, 2012 3:11 pm

Chuck Raudonis wrote: "I usually sand to 400 or even 600 grit before the oil goes on, that may be too smooth for lacquer to stick, but it works fine for TruOil or Rockhard Tabletop varnish."


I believe this quote is from one of my posts. I don't do lacquer finishes, so I don't have personal experience with adhension problems. I remember a thread some time ago concerning the relationship between the appearance of curl and the smoothness of the surface. The smoother it is, the better it looks. Someone that does spray lacquer commented that you could go too far and have adhension problems with final finish. If Mark & Greg say it's OK you can believe them, they ARE speaking from personal experience.
Last edited by Greg Robinson on Wed Jul 11, 2012 12:46 am, edited 1 time in total.
Reason: Repairing quote tags.
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Re: Highest grit sanding under lacquer

Postby Chuck Raudonis » Tue Jul 10, 2012 9:37 pm

Thanks! I did some tests on the cutoffs from the body and the bubinga didn't seem to bleed into the maple. However, I'm not going to tempt fate when I do the real thing. I'll make sure my first couple of coats of shellac are stroked with the grain starting with the maple first to lessen the chance.

Rodger, thanks for the context.

I'm in the middle of the final sand down. At 150 grit and going down!
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