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Power sanding the finish
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Author:  Wes McMillian [ Mon Sep 05, 2011 5:34 pm ]
Post subject:  Power sanding the finish
After spending much of another day sanding finishes and nursing a painful elbow that's not getting any younger, I'm seriously thinking about this now. I've already got a Dynabrade sander, but have never set up to sand finishes for some reason. (Well, I did kinda have a reason, I just today got it working right after giving up on it a few years ago.) What do y'all recommend for this? I'm shooting nitro now, leveling dry with 320 then 400. Then I go through a few grits of Micro-Mesh (wet) before hitting the buffer.
Author:  dberkowitz [ Mon Sep 05, 2011 6:50 pm ]
Post subject:  Re: Power sanding the finish
You only want to go through that coarse a grit between spray sessions. The lowest you want to go on rub out is 600 grit, and I'd start with 1000. By the time you get 400 scratches out, you'll have no finish left.
Author:  Mike Dotson [ Mon Sep 05, 2011 8:08 pm ]
Post subject:  Re: Power sanding the finish
I agree with David. 800 is my preferred after-finish grit. 600 if I have a run or something to work down. Use 400 and you might as well just put on more finish.
Author:  lespaul123 [ Mon Sep 05, 2011 8:48 pm ]
Post subject:  Re: Power sanding the finish
At my work(custom corporate jet interior fabrication) we use 320 to level the finish after 3 coats with a pneumatic power sander. After buffing about 60079596499 million cabin parts, I have begun to notice that the 320 scratches still show even after 3 coats. Since I have noticed that I now use 400 for leveling on all my guitars. I think that has made later leveling and buffing easier. Since nitro is a lot softer than the stuff we spray at work 400 grit it seems to help with "burning" as well. As for my buffing procedure I start at 1200 nothing coarser. Then I use 3000 wet and then buff with a wool pad with the 3m rubbing compound and then with a foam pad the 3m "III".
Author:  Wes McMillian [ Mon Sep 05, 2011 9:15 pm ]
Post subject:  Re: Power sanding the finish
Fair enough, as I have had the occasional sand-through. I was hopeful to start with something like a 600 when power sanding. Sounds like I may start at 800? Was just more hand sanding than I wanted to deal with, as my finish still needs some leveling as it comes off the gun.

So, again, for those power sanding - what are you using? What abrasives and grits are working best for you? How fine are you taking it before hitting the buffer?
Author:  TonyFrancis [ Tue Sep 06, 2011 4:10 am ]
Post subject:  Re: Power sanding the finish
Quote:
So, again, for those power sanding - what are you using? What abrasives and grits are working best for you? How fine are you taking it before hitting the buffer?


Unfortunately there is not really any consensus amongst builders. General advise is that you are MUCH more likely to sand through with power sanding, although the same rules apply as hand sanding. Here are some thoughts in no particular order;

Tip 1. Surface prep. The better you do you job at the foundation, the faster, better and easier the rest of the process will go. Get the bare wood and then pore filled surface as level and perfect as you hope the finish to be. Don't rely on drop fills to solve your poor prep.

Tip 2. Apply more finish where you are likely to cut through. For example; many builders spray additional coats at the edges throughout the spraying process.

Tip 3. Abrasive choice. There is a lot of confusion about sandpaper grading scales. You can level with a somewhat coarse grit by hand (P220-400) on Nitro dry. Power sanding is aggressive, and so you might go P400-600 grit wet. Why wet? Nitro is soft and very likely to clump or corn up the abrasive. Use good abrasives and change frequently.

Tip 4. When it comes to sand through's, "even" is more important than thickness. A lot of finishers cut through on their first attempts and solve this by adding more thickness. You are better to spray in sessions, leveling between, to get a even final surface. You are less likely to cut through and will end up with thinner finishes.

There are probably heaps more things I could talk about but here is my basic formula for reference (post immaculate final sanding and pore filling):

1.Seal with wash coat lacquer.
2.Tint coats if any.
3. Spray Four build coats thinned 60/40, sprayed hot (this is equivalent to the ready to spray nitro in the US, sometimes thinned 50/50 with a slower thinner in some weather conditions). I will also do an extra coat or two around any cut through prone places with the airbrush at this time.
4. Drop fill if necessary. Wait 3 days.
5. Level sand P400 about 90% level or so.
6. Spray 3 Coats (with additional airbrush around edges).
7. Wait 3 days.
8. Level sand P800 WET. I use spirit to sand as water tends to allow the finish to gum up when its so new and soft.
9. Spray 1-3 well thinned flow or flash coats.
10. Cure 1 month.
11. Wet sand P1200 or P1500. Soapy water works fine here.
12. Buff. I use Menzerna Coarse (with a bit of mineral oil) and Fine although you could just use the Medium compound. Finish by hand with 3M Finessit.
Author:  Wes McMillian [ Tue Sep 06, 2011 7:05 am ]
Post subject:  Re: Power sanding the finish
Agreed on surface prep. I don't have any (many) issues there (still have the occasional drop fill). And I always make an extra pass around all edges with the gun on every coat. I level in between sessions with 320 (as you stated, about a 90% level, don't shoot for perfection). As I stated, I'm still not getting my spray rig dialed in exactly, but it is getting better on every guitar. I've been doing the initial level on the final coat with 320. According to everyone's input, I may have been better going straight to a higher grit and putting more elbow (ouch!) grease into it.

The 800 grit level after three days is a new one, Tony. Haven't considered it. I could see that in conjunction with the final thinned coats doing wonders for the final finish, though, and make it much easier to go straight to a higher grit (even if by hand). May give it some thought. Having trouble telling from your post - are you power sanding or hand sanding? And for everyone else, no one has really answered the question - are you power sanding your guitar finishes? Or are you recommending against it?

So.....that leaves me where I'm at now. I was already figuring I would start with a higher grit when power sanding. What abrasives are y'all using for power sanding? What brands and grits work best for you? Based on everyone's recommendation, I will probably try something like 800 for starters and see if that will get me there. Hopefully, in time, my spray technique and/or equipment will improve to the point I can start with even higher grits.

What about MicroMesh discs for power sanding? I'm seeing the 1500 MM is equivalent to 700P and 1800 MM is about a 1300P. (That's a bigger jump than I realized! Although it seems to work OK.)
Author:  meddlingfool [ Tue Sep 06, 2011 9:35 am ]
Post subject:  Re: Power sanding the finish
Power sand all the way! Elseways is just too much wear and tear on the old bod.
I use a Porter Cable low profile orbital sander with 3m 30mic paper for rough, 15mic for fine, menzerna medium then fine compounds on the wheels. Buffing still sucks.
Author:  Tai Fu [ Tue Sep 06, 2011 12:44 pm ]
Post subject:  Re: Power sanding the finish
I hand sand nitro at about 1200 grit, it minimizes any and all chance of a sand through (you have to press REALLY HARD to sand through on 1200 grit), followed by 2000 grit. With a good spray gun sometimes even a 2000 grit wet sanding is enough. Use good abrasives (3M Imperial or something like it, don't use cheap/off brands that often have inconsistent grading which leaves nasty scratches. Don't ask how I know)

I use baby oil as a sanding lubricant because with water you always run the risk of the water seeping into a microscopic crack somewhere in the finish (an edge, neck joints, sound holes, there are just too many places for water to seep through the finish in an acoustic guitar). The baby oil might seem a little messy, but it gets the job done. Patience is key here and you only need a few drops of baby oil at a time, so a small bottle of oil will go a long way.

I seriously do not recommend power sanding nitro. The heat will melt the finish causing all kinds of problem. You should only power sand 2K type finishes, and even then (using Fender as an example) they usually have very thick finishes so they can just power sand away and eliminate all orange peels. Besides, when you have about 3 to 5 mils of finish to work with, even 800 grit will burn through the finish (and the wood too!) in short order.

If you find buffing tedious then invest in a power buffer of some kind. Alternatively, you may use the Stewmac foam pad mounted to a drill or drill press, which is the setup I have been using for several guitars. I plan on getting a pedestal buffer once I do set up shop though...

Guitar finishing isn't something you should ever rush on. If you want professional results you will have to invest in the time or elbow grease to achieve it. For me it took a lot of money, sweat, and elbow grease.
Author:  Rod True [ Tue Sep 06, 2011 1:00 pm ]
Post subject:  Re: Power sanding the finish
Tai Fu wrote:
I seriously do not recommend power sanding nitro. The heat will melt the finish causing all kinds of problem. You should only power sand 2K type finishes, and even then (using Fender as an example) they usually have very thick finishes so they can just power sand away and eliminate all orange peels. Besides, when you have about 3 to 5 mils of finish to work with, even 800 grit will burn through the finish (and the wood too!) in short order.


Where do you get this stuff? People have been power sanding nitro for years. It just takes practice. If done right there is very little heat from sanding, even power sanding... How do you think the guys out there who spray guitars for a living do it? Addam Stark puts out one of the best finishes on the market, does a lot of custom builders work. He power sands his nitro finishes, otherwise they would never make any money at it....
Author:  Tai Fu [ Tue Sep 06, 2011 1:21 pm ]
Post subject:  Re: Power sanding the finish
Sorry... I only speak from logic. I am sure it could be done if done right, but I wasn't sure what kind of equipment or papers to use. I just know those palm sanders (the one that takes standard sandpapers and works by vibrating) doesn't give an even enough finish for me to trust it on a nitro finish.

Can anyone shed some light on this subject?
Author:  Chas Freeborn [ Tue Sep 06, 2011 1:25 pm ]
Post subject:  Re: Power sanding the finish
It's worth paying attention to the sander too. I use the small orbit (3/32" I think) Dynabrade when sanding finish and tread very gently.
My favorite sanders in the world for precision work, including finishes are the little Nitto line sanders. Model # LS 10 as I recall. Straight line sanding leaves no swirl-marks, the Velcro interchangeable pads can be contoured to specific tasks, light, quiet and don't take much air to drive them. Not cheap, though...
-C
Author:  Wes McMillian [ Tue Sep 06, 2011 1:38 pm ]
Post subject:  Re: Power sanding the finish
Thanks, Chas. I do have the Dynabrade sander already (although I think mine is the larger orbit). Any recommendation on abrasives for finish sanding? I'm thinking about trying the MicroMesh set, but I'm wondering if just some quality wet/dry discs would make more sense. I'm guessing just a couple of grits of wet/dry would have me ready for the buffer. Shame to spend the money for a full set of MicroMesh discs that I won't use.

As Rod said, I know it can be done. I could just go at it, but I was hoping to order what I need tonight and I'm looking for recommendations. Any other suggestions on materials or technique are greatly appreciated, though!

Tai Fu, I've got a power buffer setup that I've had for years. I'm not afraid of investing the time or elbow grease into a finish, just always looking for ways to improve the process. Hopefully we all are.
Author:  TonyFrancis [ Tue Sep 06, 2011 5:13 pm ]
Post subject:  Re: Power sanding the finish
Wes,

There are several great sandpapers to use, and it depends what sander you are using. For a jitterbug such as the PC Speedbloc or Nitto air sanders you will be buying sheets. For dry sanding the 3M Tri-M-Ite or 3M gold 216U(?) are good choices. And you can use regular wetordry sheets! Obviously a huge advantage of these sanders is that they use the same paper as sanding blocks so you save on buying multiple forms of the same abrasive.

For ROS sanders, Mirka Abranet is the best I have found for leveling dry. Most standard papers just clump up instantly making them useless, although there is now the 3M Purple Imperial discs. For sanding wet, Micromesh is was basically the only way to go, although now there is also the 3M finishing film! Haans also reccomened the Deer brand sandpaper for wet sanding.
Author:  Fred Tellier [ Tue Sep 06, 2011 5:53 pm ]
Post subject:  Re: Power sanding the finish
I have used products from Joest abrasives http://www.joest-abrasives.de/ the pad is foam backed and velcro attached to the RO sander. I posted a while back on availability of these products on this forum http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=28636&hilit=joest+sandpaper I deal with Robert who's phone number is posted in the topic his prices are real good.

These products allow the Vac. pickup of the dust and do a great job in all parts of the build, but are really great in dry sanding a finish, starting with 600 and up to 2000 or even finer. A buffing with a power buffer creates a great finish.
Author:  Tai Fu [ Tue Sep 06, 2011 8:18 pm ]
Post subject:  Re: Power sanding the finish
That's good to know... as I am planning on doing luthiery for a living anything I can do to speed things up would be helpful when I will be making over 20 guitars a year...
Author:  Wes McMillian [ Tue Sep 06, 2011 8:30 pm ]
Post subject:  Re: Power sanding the finish
Thanks, Tony and Fred. Fred, I guess you're using a hook and loop pad from your post? Tony, you supported the Micro Mesh for ROS sanding wet. I assume the MM is just the same product that the sheets are but in disc form (not hook and loop). Should the pad that comes with the Dynabrade be good enough?
Author:  TonyFrancis [ Wed Sep 07, 2011 4:26 am ]
Post subject:  Re: Power sanding the finish
Wes McMillian wrote:
Tony, you supported the Micro Mesh for ROS sanding wet. I assume the MM is just the same product that the sheets are but in disc form (not hook and loop). Should the pad that comes with the Dynabrade be good enough?


Wes, Micromesh is available in discs from a number of suppliers like Lee Valley but also from the manufacturer. The Dynabrade pad is one of the best, and you can get other pads and density's for it. Usually the harder the better, but some people use soft pads with great results.

I would avoid any foam backed paper for leveling as they really just dull the surface and do not level it properly. But they are fine for the finest grits. If you read Frank Fords FRETS article on sanding blocks, you will see he recommends hard blocks wherever possible. Any power sander is sort of the opposite of this.
Author:  Wes McMillian [ Wed Sep 07, 2011 6:10 am ]
Post subject:  Re: Power sanding the finish
Thanks for the help, guys. I wound up trying the Abranet discs for leveling up to 6oo, then Micro Mesh 1800x and 2400x to get to the buffer. I'll try going straight to the MM on initial finish sanding. If that doesn't work out, I'll hit it with the 600 first. I'm sure, as with everything, it will change over time and experience, but this is a starting place. Driven partly by what Lee Valley had in stock (wanted to try some fish glue too).

We'll see what kind of life I get out of the Micro Mesh in this application. Haven't worn out any sheets yet.
Author:  Phillip Patton [ Sat Sep 10, 2011 10:59 am ]
Post subject:  Re: Power sanding the finish
Wes McMillian wrote:
After spending much of another day sanding finishes and nursing a painful elbow that's not getting any younger, I'm seriously thinking about this now. I've already got a Dynabrade sander, but have never set up to sand finishes for some reason. (Well, I did kinda have a reason, I just today got it working right after giving up on it a few years ago.) What do y'all recommend for this? I'm shooting nitro now, leveling dry with 320 then 400. Then I go through a few grits of Micro-Mesh (wet) before hitting the buffer.


I'm in the middle of spraying a build right now, and I just leveled the the lacquer for the first time. Yesterday I sprayed four coats, and today I leveled with 400 grit with my dynabrade ROS. No sand throughs. I'll keep leveling after each round of spraying, then after the last round I'll level with 1000 grit, probably wet. Then I'll use micromesh sheets by hand before buffing. I've been using these discs: http://www.abrasivesupply.com/5in_PSA_D ... _s/115.htm

Pretty pleased with it so far. Last time, I did the final leveling with the 400 grit, and there are still some swirls in the finish. Hope to avoid that this time.
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