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I always seem to have a problem with tiny swirl marks being noticeable in my buffed out finish ( EM 6000) . I only use a random orbital sander and am wondering if that might be the issue. I sand the bare wood with 120 followed by 220. During the finishing process, I will sand back after five or six coats and after 10 or 12 with P600. My final sanding is usually done with the following sequence. P600, P1000,P1500, P2000, P2500, all done using a random orbital sander. I must be doing something, ( maybe several things ) wrong. I would appreciate any advice anyone might offer on this problem. Thanks.

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Under Compensated Nut!

I only used ROS once, similar problem. Difficult to judge when surface completely finished at each grade.
I went to hand sanding (to P1500, then buff), each grade at 90 degrees, last with the grain - easier to see when finished.
I look forward to the replies from ROS users

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

On the bare wood sanding - 120, 150, 180, 220 grit. Unless the bare wood is really rough, you can start at 150, and maybe go to 280 grit. With a ROS, you need to have controlled, but light hand, and clean the discs with an abrasive cleaner often. If you are running at high rpm's, the discs will get hot and clog quicker, especially if you are sanding resinous woods. This can cause deeper swirl marks.

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

With ROS sanding you need a vacuum hook up. Also with a new disk you should have a sacrificial piece of wood that you hit for a second or two to knock down the high grit particles before going right to your top or back. A # 2 pencil or softer used at a steep angle to make a series of diagonal cross hatchings across the whole surface will let you know where you are sanding and how much you have sanded. Don't use the point of the pencil or use it cross grain or with the grain, only on the diagonal and with a light touch. I use this trick for all of my woodworking sanding. If the lines are about 1 1/2" a part that is fine. This is akin to using chalk or pencil to mark a dovetail to show what surfaces need to be removed or using machinist bluing to show surfaces that need to be worked or felt pen on frets for crowning and on and on. Try it, you will like how it informs your sanding.
L.

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Cut to size.....Beat to fit.....Paint to match.

These are all great tips!

Before I tried to make guitars, I knew how to sand, just as I knew how to sharpen, etc. I found building guitars was much more demanding. Instructions about sanding for general woodworking emphasize using the right grades in the right order. What's left out is the challenge of getting the surfaces truly flat, or accurately curved without any extra dips. Any hollows will show scratches. Most of the methods above address this problem. With or without use of a ROS, it has helped me to check the surface for flatness by using pencil cross hatches and sanding with a hard block I have confirmed to be flat. Correct hollows before moving to a finer grade.

In terms of the "light touch" with the ROS, for me this means no hand pressure--let the weight of the sander do the work. Also, be careful to glide the sander into at the correct angle, or even put the sander on the work before you switch it on.

Breaking in a disc a little to knock off any raised grits sounds like an excellent idea. Also, though, throw away sanding discs once they start to wear noticeably. Scrubbing away with a half-worn-out disc is just asking for trouble.

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Tim Allen
"Never hurry, never rest."

I'm wondering whether part of the fault might lie with my Porter-Cable ro sander. I have had for quite a while, so it may be in need of service/replacement. The swirl marks always seem to be in the finish, not the wood. Generally, I fill porous woods as per Todd's youtube video. And that doesn't appear to have swirl marks. So, for those who use an ro sander on the finish coats, what sequence of grits do you use?

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Under Compensated Nut!

If the orbit is much greater than 1/8" (3mm) it can be very difficult to get rid of the scratches from low order grits. I don't know the spec. of the Porter-Cable you have, but that might be the problem. Might be time to get a good, new, finishing palm sander and reserve the PC for other duties.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

The swirls are likely coming from "seeds" that get caught up in the paper. This is partly because the mechanical sander generates more heat and softens the coating more. Auto body guys use air powered RO sanders and generally use plenty of water to float away the debris. But that flood of water is not a good thing when sanding guitars. If you wish to use a ROS you may consider only doing your first few grits with it and using a good lubed or sterated disc. Mirka makes some very good discs, both the abranet and autonet are great for sanding out finishes. I would still do my final 3-4 grits by hand using varied patterns to see what I was doing and make sure that I got everything sanded proper. Guitars get scrutinized closer than almost anything else you will ever finish.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

IMHO your going from 120 to 220 grit, I'd add 180 grit in the middle, I don't think you'll successfully remove the 120 scratches with 220 paper and as has already been said RO's are harder to see what your doing.


~Nil carborundum illegitemi~

P 2500 is just a little finer than cami 1000, so you may want to use a little more aggressive polishing compound initially, then go to the buffing compound.

Thanks for all the input, folks. Really appreciate it.

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Under Compensated Nut!

Everything I have read here is spot on. Sanding EM 6000 does not require a ROS. Sponges, blocks, and paper all the way too 4000 grit. I start at 320, dry. Then the rest is with mineral spirits. 400, 600, and up. Mineral spirits will in no way react with EM 6000. Water can. DNA most def will.