Gunbei, yes this is a very safe method for use on veneers. Because the pumice and rottenstone are so fine an abrasive, you are not nearly as likely remove more than thousandths of an inch of anything, whether it be a varnish finish or wood. What you are really doing is polishing - not "sanding". Be extremely careful near corners and joints, though. I have found the joinery on most speakers (even $15K+ speakers) to be highly suspect, at best. Not all mind you. Sonus Faber is one exception, though their sound is not my cup of tea, they are generally well built. Anyway, the joints, especially mitering at the corners, is often slip-shod. Where there is not a perfect fit, the miters are often "rounded over" to give the illusion of a better fit. In these instances, the veneer becomes very thin and can be only paper thin above the underlying composition board. This is less an issue with the pumice/rottenstone process than using sandpaper when refinishing, but it pays to be extra careful.
As for nicks and scratches, this is an obvious problem with veneers due to their thickness (or lack thereof). VERY shallow scratches are best sanded out (CAREFULLY) using a sanding block to ensure even removal of the surrounding wood. You should try to determine just how thick is the veneer on your speakers BEFORE attempting any sanding of anything more than superficial scratches. If you go through the veneer at all you will be most sorry! That being said, most veneers will be thick enough to allow you to sand out most scratches.
Dings, if they are not TOO deep can sometimes be lifted using water and heat. First, I should make a distinction between dents/dings and nicks. Dents/dings are where the wood is compressed, though none is "missing". Nicks are where a piece of wood has been physically removed. I have used this method successfully for dents/dings many times on both veneers and solid wood. Nicks, on the other hand, need be filled. I would suggest this only if you feel comfortable working with wood. For a small ding, take a paper towel, fold it in fourths and saturate it in water. Get an old soldering iron (an iron not a gun) and heat it up. The electric ones work fine but fire heated ones work, too. Lay the wet toweling over the ding. Press the hot iron on the toweling directly over the ding. Try to position the iron as much as possible to only cover the area of the ding. The water in the towel will create steam and will hiss and pop. Apply pressure for just a few second, then remove the toweling and check for effect. Often, only a couple tries will swell the fibers and grain and "lift" out the ding. Be extremely careful not to dry out the towel and cause a fire or a scorching of the wood.
The other alternative for dings, of course, is filler. You can buy fillers to match. But, if you have sanded out scratches be sure to save all the "sanding dust". Put it in a small jar or can and mix in a drop or two of Elmer's woodworkers glue. Use this to fill ding holes and you will get a very good match. If there is a distinct or highly contrasting grain pattern, you can some times follow up with a bit of stain to match the darker grain applied with a very fine tipped artist's brush.
Both the sanding and "sweating" methods will entail a refinishing of the wood and should be used only when you are comfortable with the subsequent process of REFINISHING the wood. That almost always means the entire speaker, not just the repaired area. As with a lot of DIY projects, what sounds easy can be less so in application. Maybe practice on some wood that is of no importance first to see if you feel comfortable tackling those rosewood Proac's, not to mention someone's $20K floorstanders!
As for nicks and scratches, this is an obvious problem with veneers due to their thickness (or lack thereof). VERY shallow scratches are best sanded out (CAREFULLY) using a sanding block to ensure even removal of the surrounding wood. You should try to determine just how thick is the veneer on your speakers BEFORE attempting any sanding of anything more than superficial scratches. If you go through the veneer at all you will be most sorry! That being said, most veneers will be thick enough to allow you to sand out most scratches.
Dings, if they are not TOO deep can sometimes be lifted using water and heat. First, I should make a distinction between dents/dings and nicks. Dents/dings are where the wood is compressed, though none is "missing". Nicks are where a piece of wood has been physically removed. I have used this method successfully for dents/dings many times on both veneers and solid wood. Nicks, on the other hand, need be filled. I would suggest this only if you feel comfortable working with wood. For a small ding, take a paper towel, fold it in fourths and saturate it in water. Get an old soldering iron (an iron not a gun) and heat it up. The electric ones work fine but fire heated ones work, too. Lay the wet toweling over the ding. Press the hot iron on the toweling directly over the ding. Try to position the iron as much as possible to only cover the area of the ding. The water in the towel will create steam and will hiss and pop. Apply pressure for just a few second, then remove the toweling and check for effect. Often, only a couple tries will swell the fibers and grain and "lift" out the ding. Be extremely careful not to dry out the towel and cause a fire or a scorching of the wood.
The other alternative for dings, of course, is filler. You can buy fillers to match. But, if you have sanded out scratches be sure to save all the "sanding dust". Put it in a small jar or can and mix in a drop or two of Elmer's woodworkers glue. Use this to fill ding holes and you will get a very good match. If there is a distinct or highly contrasting grain pattern, you can some times follow up with a bit of stain to match the darker grain applied with a very fine tipped artist's brush.
Both the sanding and "sweating" methods will entail a refinishing of the wood and should be used only when you are comfortable with the subsequent process of REFINISHING the wood. That almost always means the entire speaker, not just the repaired area. As with a lot of DIY projects, what sounds easy can be less so in application. Maybe practice on some wood that is of no importance first to see if you feel comfortable tackling those rosewood Proac's, not to mention someone's $20K floorstanders!