Sandpaper theory

[deadedith]

Let's say you are sanding a top.

You've sanded to 220; you cut a piece of 320 and run it once the length of the top.

On your next run, is that paper still 320? If so, at what point is that paper doing more harm than good? I know it will continue to sand at less effectiveness with each stroke, and that vacuuming between strokes - both the board and the paper - helps a lot. But to do the absolute best job of sanding you can do - when do you toss a piece of sandpaper? Or is it even that important?

Thanks

[tippie53]

I don't sand past 220 but my final pass is done across the grain. I know it goes against all we were taught but it won't leave marks. That also helps to flatten to top so you don't have corduroy . If you sand the top too much with the grain you will have that happen.
Also USE A BLOCK. When you get to the finish part , it is all about the prep.

[deadedith]

One of the good things about the tru-oil is that you can basically polish your guitar BEFORE putting on the finish.
To expand on that: there is no adhesion problem with the TO. Some people sand to 12,000 before wiping on a drop of the stuff. I go up to 1,000, then apply TO - a very thin coat, let dry 3 hours or so, lightly buff with 0000 wool, apply another coat and so on.
I am basically donem though from that point you can put it aside for a few days and then polish to a glassiness same as any other finish, or, as I do, actually take the shine down to a silky sheen with a white scotchbrite pad.

My original question had to do not so much with the grits involved, as to basic info about how long your paper should be used - I use the 3M Fre-cut from Stewmac.

[Kevin Sjostrand]

That also helps to flatten to top so you don't have corduroy.

John, I was wondering what that was called; corduroy.
I got that on my last build. I was not particularly bothered by it, but as the lacquer continued to cure (and shrink I suppose), it showed up. I will try the sanding across the grain next time.

Certainly the longer you use a piece of sandpaper the less effective it is going to become. This will happen sooner if you are sanding a hardwood verses a softwood. The grit breaks down, and loads up. However, I don't know that it changes grit....such as the 320 becomes 220, or even 400. I believe you will just have a less effective piece of sandpaper the longer you use it. If you start seeing scratches in the surface deeper than what you started with, then you best throw that piece of sandpaper away, and get a fresh piece, it is probably contaminated.
Also, the cheaper the paper (in general), the more chance you will get an un-uniform grit surface.
It is a good question, don't assume your paper is doing its job....inspect your surface as you go.

Kevin

[kencierp]

I use a Porter Cable RO sander (any high quality RO will work) for the top and back prior to applying finish --- think about it -- its the best option since it sands flat in "all" directions. I've never had a problem with grain ripples -- also I never wet the wood before sanding which seems to be a popular suggestion with which I respectfully disagree. $.02

[tippie53]

I use a RO sander . I agree that is the tool . Love it

[Tony_in_NYC]

I think there is too much thought being put into this. When I am using a piece of sand paper, it is wrapped around a block of some sort. That means that the entire surface of the paper is not being used, right? What I do is to periodically rub my finger on the part being used and compare its roughness to the part not getting used. When they feel pretty different, I change to a new piece. Its simple. I also use my shop vac to suck the dust away and clean the paper as well, but not every stroke.
I want an RO sander for finish prep. Someone tell me wife.
Do either of you, Ken Cierp or John Hall, use the RO with wet/ dry paper for level sanding and polishing, or do you think it is too dangerous for that purpose?

[tippie53]

I wet sand by hand with a block. I use the RO for in the white and level sanding but not for finishing. Just the way I do it.

[kencierp]

If the sandpaper clogs that is a problem and it should be replaced -- I never give the grit loss a thought I just throw it away when it does not cut as expected.

I use pink or blue insulating foam for my sanding blocks.

Yes I level finish wet and dry with a RO

YES leveling with a RO is risky -- you can be back to square one in an instant. I've mentioned this before I believe the average new comer simply does not put on enough material -- even when working by hand -- cut through is problematic if the coating is too thin at the offset.

[Tony_in_NYC]

Got it.
Brushing on the lacquer means I do not get orange peel like you sprayers. I get massive ridges! And by massive, I mean bigger than orange peel type stuff. Getting the finish almost to level with an RO would be a big time saver vs. doing it all by hand. I find myself reaching for 320 grit paper to get the ridges low and I worry about sand through. So far, so good on the claro guitar and I had zero sand through on the Koa OM, but there were some scratches from the 320 that I had to work at getting out and I only saw them when I was up to 8000 grit micro mesh. That was a bummer....having to go back to the lower grits in some areas when I was so close to polishing!