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Wet Sanding With Mineral Spirits


Brian
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Just snatched this off another forum thinking everyone here would benifit in reading the thread.

Stew

I've just recently heard of someone using mineral spirits to soak their wet/sandpaper for between coats. Would this further remove color and increase rish of sand throughs?

telecast

Not if you're using lacquer or shellac. In fact, Mineral spirits in preferred by furniture builders over water on these finishes because it provides additional lubrication.

Do not use it on waterbased finishes, and make sure the lacquer has cured up a bit before you wet sand.

Drak

Another plus is that it has less chance of seeping under and lifting the nice lacquer finish you just shot. Water can and will do that.

jrfrond

When I worked at Steinway Piano in the 70's, I was in the rubbing/polishing department (great training grounds!), and we used mineral spirits mixed with linseed oil, specifically for the lubrication properties, during the initial wet-sanding. Subsequently, powdered pumice, and then later, rottenstone was added, and the finish was completed with these "compounds" and felt buffing pads. These were key steps to the highly-revered Steinway Piano finish, which later on, would find it name, at least, on Yamaha drums. We did this finish on the late Jeff Porcaro's Camco drums at the time, right at the factory.

Today, I only use mineral spirits on nitro finishes. Otherwise, with synthetic lacquers, I use water with a little Dawn dish detergent added as a lubricant.

The downside of working with mineral spirits is that it must be in a ventilated environment.

telecast

Jfrond's reference to Pumice, Rottenstone, linseed oil and mineral spirits are what woodworkers refer to as a French Polish, or less commonly a French Rub. A lot of work!

Stew

Cool. Because I'm working with nitro lacquer only and it seems that I'm having trouble sanding through to the primer using water no matter how careful.

Should I let the sandpaper soak overnight in the mineral spirits? After use, should I wipe off all excess and let evaporate and dry for 24 hours or more? And this will not interfere with additional coats or should the mineral spirit stage be after the final coat has been applied? Do I still plug the screw holes with puddy to prevent paint lifting?

jrfrond

First of all, stop using sandPAPER, and start using abrasive cloth, like Micro-Mesh. It's more expensive, but WELL worth the effort. They are extremely flexible, and get wet in a hurry, plus you can get down to 12000 grit. At THAT grit, if you've done a good job, you can go right to swirl-mark remover and glaze.

Actually, those ARE the ingredients for French Rub (French Polishing uses no abrasives, just shellac, oil, and mineral spirits). The French Rub is different from the sanding in the fact that the shellac is applied with a pad, and later, oil and rottenstone are added, as the shellac hardens, to bring up a high-polish shine. THIS is an art form, for sure!

Stew

Where can I get micro mesh? I had been using 600-800-1000-1200-1500-2000. I usually, if I don't get any sand throughs, get a nice mirror finish. After the 2000 wet sand, I use a swirl remover by 3M (removes down to 1500 size scratches) and then polish.

Thanks again.

telecast

It sounds to me like you're not getting enough product on the guitar. There's no reason to sand through with PAPER if your doing it right. I use 400 after the first few coats, then 600 after the next few, 1,000 just before the last coat, and 1,200 before buff out, then two grades of buffing compound.

Oversanding is a common problem when you're getting started. Learn how to apply the correct amount, and the subsequent tricks, like don't spend too much time on corners.

Stew

I'm following the rules of three: 3 coats per day for three days. I don't want to go too heavy so I make light passes.

Another problem, how important is it to get all the shiny finish flat between coats? When I start wet sanding between coats, I'm using 800 grit, and there may be small, very minor, shiny areas. If I spend too much time at that one area, say near the horn/cutaway area, I start to get a little primer peaking through my color coat. Keeping in mind, the surface is flat (in color and in level) but the shiny parts are about the size of ground pepper.

I suppose the next coat would cover that anyway so I'm assuming a general level wetsand with some minor shiny specs is not an issue, unless this is your final wetsand before buffing out. Right?

Telecast: I know what you mean about those darn corners. I just barely hover those points when I'm sanding. And I make a point of being very careful not to bend my paper over the edges when I'm sanding the flat portion of the side body.

telecast

Ok, let's address this one at a time:

"I'm following the rules of three: 3 coats per day for three days. I don't want to go too heavy so I make light passes."

What do you consider a 'coat'? A coat of finish is applied fairly heavy without runs or sags. It flows out and leaves a gloss finish, with no dusty or orange peel look. Your three coats may not even be adding up to one real coat.

"I suppose the next coat would cover that anyway so I'm assuming a general level wetsand with some minor shiny specs is not an issue, unless this is your final wetsand before buffing out. Right?"

Right, to a degree. If you're shooting lacquer or one of the newer waterbased products, each successive coat actually burns in, or melts in to the coat before it. Everytime you shoot a coat, you are building up the finish. When you sand it down, you are removing some. Ok, that part is a given, but those low spots that look like the head of a pin are filling in, because you didn't remove any from them, just the surrounding area.

Your first coats should be 'knocked down' only, not sanded smooth. Then shoot it again but sand it a little smoother. Keep this up until all those spots are filled. Also, you will greatly reduce those spots by getting proper flow-out when you apply the finish. This goes back to those heavier coats.

"And I make a point of being very careful not to bend my paper over the edges when I'm sanding the flat portion of the side body."

If you are sanding the flat parts of the guitar without a sanding block, stop. Get one, they're inexpensive, help you control sand throughs, and will improve the looks of your finish by tenfold. It'll also stop you from wrapping your paper over the edge when you sand. Just hold it flat on the body and let the block overhang the edge while sanding.

As far as corners go, if they're real sharp I don't even sand them. You'll never notice after buffing.

Stew

Yeah, I might be removing too much between coats. I always feel that when I'm wet sanding down after my final coat that I may run into problems once I have reached the 1200 grit stage. The directions I've followed are from the Reranch website recommending 600, 800, 1000, 1200, 1500 to 2000 then buff out.

I think I would feel safer using the 1200 grit for my final after shooting my last coat.

Thanks.

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