Wet sanding polyurethane b/t coats w/ mineral spirits lub


I have read that this is an effective technique. More specifically, the book said to use the type of thinner/cleaner for the finish you are sanding as the lubricant for waterproof (Silicone carbide) sandpaper. I am not sure that I can bring myself to do it without being re-assured.
What would be the technique to do this. Do you need to rinse it off? Would you rinse that with straight thinner, or water, or a dry rag, or any prescribed order of any or all of the above? Does it require a wipable surface (e.g. tabletop) to squeegee off?
Assuming the following (another reason) were true, is this safe?
Assuming the following (another reason) weren't true, is this safe?
I think I just thought of another reason why not to. I am doing drawers. This may trap slurry, swarf, whatever you call it, into tight areas you can't scour, squeegee, gravity won't work, etc.. This trapped thinner may be harmful over time, regardless of the other answers.
BTW, all the drawers now have an "air tight seal" around all meeting corners now that the bottom and sides are poly'd to one another after the first coat. Also, I have and will be using a tack rag to clean it. I foresee spotlessly clean corners regardless in my near future. Recommendations? I'm using Minwax Spar Satin Poly. I am going to be sanding with 220 (Silicone carbide) sandpaper anyways. Just wondering if MS as lubrication is safe, and now, in this drawer situation.
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The rational behind this is to avoid leaving a non-compatible residue on the finish, while getting the benefits of wet sanding. If you use water, on a oil based finish, you might trap some in an imperfection somewhere.
But before you try mineral spirits, consider the down side. It is flamable. You should not be doing this inside a shop, unless you have a professional finishing room. You should have a fire extinguisher handy, and be wearing gloves. You also should not be using it if it will soften the finish you are sanding. If you try laquer thinner to sand laquer, you end up with goo. It is not a great rule of thumb. But to be fair, mineral spirits is not near as flamable as laquer thinner or acetone. But caution is called for.
You do need to rinse, with mineral spirits, and let dry, and then use a tack cloth. Water will work almost as well, but you do need to let it dry a bit longer. That is the benefit of using mineral spirits. Mineral spirits in it self will not leave a residue. And is useful for dewaxing if a finish might contain disolved wax. Just do not use it inside.
bent wrote:

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The mineral sprits are as flammable as those which serve as a solvent in the polyurethane, assuming that we're speaking of oil-based poly.
The rationale is that wet sanding keeps the finer grits cutting longer, but they should be wiped and tacked thoroughly. Dry wipes ok, since the mineral spirits evaporate rapidly.
Stearated open coat papers are available if you don't want full cutting. The minimal soap that lubricates them dissolves in the next coat of finish.
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Is there more than one kind of mineral spirits? I never could get it to burn......years ago I needed to scrub a 10,000 ft concrete floor with the stuff, had reasonable paranoia about igniting the place so I tried various tests...flammable it wasn't, at least not with the expected normal environment for the job. If memory serves me correctly even a propane torch wouldn't light the stuff (will have to check that one out again).....Rod
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Why are you using poly and spar at that on the drawers anyway? Is this for an outdoor application. They will smell for a long long time and since it is spar they will never harden completely. Most people recommend shellac for drawers. Cheers, JG
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Many people use thinners or water to wet sand between coats, but all you really need to do is break the glaze. A red 3M pad will do beautifully for this, with no lubricant. 0000 steel wool will work very well also, but it sheds and you have to be very careful to remove all of the slivers before the next coat goes on or you do a great decoupage job over them. I only wet sand the final coat unless I somehow get some big globs of stuff like dust or bugs in the build up coats. Small dust nibs can be left as they are in the build up coats and simply knocked down in the finish coat, but small is a difficult term to define. Typically, things like mosquitoes are too big to leave...
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Okay, here's what I do. First off, I thin the poly. Maybe, about five percent. The stuff is barely thicker than water, with the thinner. (I swear, one of this days I'm going to try dipping a small project, rather than brushing it on.)
I put it on quick, and LIGHT. I use as light a touch as I can and still get full coverage.
It's more of a sealer coat, at this stage. I let it dry thoroughly, usually overnight, and then sand lightly with 220 grit sand paper.
Then I apply a full strength coat of poly. Again, I don't fuss with it. I apply it lightly as I can, since sags and runs are practically impossible to remove. Another sanding with 220. It should be very smooth at this point.
Usually, I apply another coat. Again, it most dry thoroughly. However, this time, I use 600 grit sandpaper. It doesn't sand it as much as it polishes it.
It's going to take at least two coats, maybe three. Get use to the idea.
For sure, patience is not my strong suit. But dry means thoroughly dry, not mostly dry. If it takes overnight, that's what it takes.
Do not fuss with poly. It flows together as it dries.
A couple of thin coats are always better than one thick coat.
That's my story and I'm sticking to it.
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Don't laugh, I've done this recently with knobs. Dip, let them drip, then slowly rotate them to distribute any excess (less than a drip) around the knob.
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I am about to sand the first coat w/ 220 SC. It looks like good coverage. It was easier than I thought to control thickness, flow, and globs. It is great. I am pretty good w/ applying finishes, and polyfilla. Takes some time to learn. I can be trusted to do it the way it should be done. Of course it raised the grain, and it has the speckled appearance where it looks like sand at the bottom of millions of whirlpools. I am not sure how much MS I would need to effect the advantages, and since it is the inside 5 of drawers I probably won't chance - ok maybe I'll flip it on its side and try MS. I think I will use three coats of poly. This is the most protective. Spar, yes not the hardest, allows for movement, stops cracking, and I have put about 400 ft sq. of it on my picnic table and it gets hard enough. Not for floors though. Do not use high-gloss to use outdoors. I'll report back.
w/r/t to final finishing, don't know who posted w/ 600 recommendations, but I'd like to know more. For now, just specifically about this project. I know I am going to want to do something! I've got 220, 320 400, 600, 1000, 1500, & 2000 SC papers. And 3M rubbing compound fine. These drawers are white outside including an extremely accurate chamfer on the inside of the top where the edge meets the inside of the drawer. Looks awesome. You can only tell there is a lock joint on half of the sides. However, there are gaps at the bottom to sides - it is NOT air tight. Most of the insides is.
Do you need to wet sand (as opposed to dry sand) if it is after the final coat? Would you need to do more - like wax. I've got a messy Johnsons furniture polish wax that is the colour of buffing wax avail at Lee Valley. must be nearly the same.
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Probably too coarse. Remember, there's a difference between leveling and toothing, and use a semi-rigid block under the paper to level. One of the _many_ reasons, most of which have been embedded in my sking, that I don't use flexible stuff like steel wool.

The reason(s) you sand are to level and to provide a mechanical hold for the succeeding coat. I can't think of any reason why you'd want to sand after the final coat, if that coat is properly applied, unless you're trying to cut the gloss. Wax will cut the gloss a bit because of it's polymer size, too.
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