2000 grit on Poly

[Dan White]

Hi. I've been experimenting with different polyurethanes for my butcher block counter tops and it looks like I'm settling on a Sherwin Williams clear poly. I want a pretty high shine, but not the tacky gloss feel and uneven "liquid" look. My understanding was that I should sand the final coat with maybe a 400 or even as low as a 220 grit, and then go up to 600 and higher depending on what kind of finish I would like. Tonight I decided to try wet sanding with 2000 grit on the clear gloss poly right off the bat instead of going through the paces of higher and higher grits (this is on a test piece). It looks to me like it does fine. I have a nice smooth finish that has a little less gloss but is smooth to the touch and I don't see any imperfections. Am I missing something on my final 2000 grit finish by doing things this way? Might it be a better finish by going something like

400/600/1500/2000?

I sanded with 2000 on one half and left the other half of the sample with the high gloss "tacky" feel. When I put a wet towel down to clean off the sanding dust I noticed that the water beaded up on the untouched poly while it layed down on the sanded part. I can understand why this might be, I suppose, with the micro abrasions on the sanded side, but I wonder if it has any impact on the resistance the finish has to liquids. After all, I'm doing all this in order to protect the wood from occasional spills.

Any comments or ideas are appreciated.

dwhite

[mp]

You're probably spending a lot more time sanding than you need to.

Try a coat of butchers wax. It'll make the surface less porous (easier to clean) and restore the beading.

[James "Cubby" Culbertson]

Hiya, I can't speak to the question at heart as I don't use the poly stuff. Have you considered other finishes that are a bit easier to repair? Kitchen countertops will no doubt see a good number of "accidents" and you may want to consider the possibility of having to re-finish/touch up areas down the road. My understanding is Poly is a very tough finish but repairing it is even tougher. Cheers, cc

[George]

OK, great. Now you've learned a bit about surface tension. Doesn't mean a thing as long as the film is continuous.

Points out something others are trying to say when they tell you not to use polyurethane. Any break in the surface is going to grow because moisture will use the same effect to slide between the finish and the wood, then into the wood, promoting further separation of the two surfaces.

Makes thicker better, but of course, that promotes the "liquid" look.

[J. Clarke]

True for any finish, but the effect that he's seeing is called a "water break free surface" and the main thing that it indicates is the the surface is free of oils, waxes, etc. Checking for water-break free is one of the steps in adhesive bonding in many critical applications in aerospace. Essentially he cleaned off any wax or oil that evolved during the curing of the finish.

I can't see where that is any reason to use a less durable finish in a situation that is going to be subject to a good deal of heavy use.

[J. Clarke]

What finish is "easier to repair" that is going to last for any time in a wet environment with exposure to acids and bases and the occasional solvent?

[Mike Marlow]

No. The only reason to use more abrasive grits is to more quickly knock down areas. If your application is smooth enough that you can achieve the desired smoothness with only 2000 grit, then don't go any more abrasive - you're just making more work by doing so and removing finish for no good reason.

As long as you don't go through the finish - which is unlikely with 2000 grit, then you are not at any risk with what you see. You just don't have the same surface tension as when the scratches weren't there. If it really starts to bug you then go get a quart of automotive buffing compount (light cut) and buff it up with that. It should take the fine scratches from the

2000 right out with a bit of elbow grease.

Comments? Here?

[Norm Dresner]

Sanding first with the very fine abrasive gives you a very smooth but not necessarily flat surface that you'd get by leveling with the heavier grits first.

AIUI surface tension is affected by the roughness of the surface so it's not at all implausible that your sanded side exhibits a lower surface tension than the rougher side.

As long as you haven't sanded through the finish, you have the same material on both sides, though not necessarily the same thickness. My gut reaction is that your finishes provide equal protection on both halves.

Norm

[Charles Spitzer]

isn't the flatness of the surface affected not by the grit, but by the flatness and inflexibility of whatever is behind the sandpaper? the paper would only take off the high spots if the backing was hard and already flat.

[Dan White]

This is a commercial application and the counter will be subject to occasional water/coffee spills and I needed something that would provide the best protection from stains. The areas that will contact food will have mineral oil on it.

thanks, dwhite

[Dan White]

That's interesting. I suppose that might also explain why the sanded part feels smooth while the unsanded gloss poly feels sticky to the touch. Maybe there are oils on the surface.

Poured some hot coffee on both surfaces today and wiped it up after about

1/2 hour. Both sides came clean although the sanded part looked like it wasn't going to come clean at first. In no time it did come perfectly clean. I couldn't find any hint of coffee. I also put a hot pot of coffee on the poly for about 5 minutes. It looked fine. I did see what were almost microscopic marks or bubbles when I removed the pot, but didn't have time to see exactly what that was. I'm going to try that again tomorrow, but things look good so far.

In a prior trial with Minwax, the coffee did penetrate to the wood but I'm pretty sure that's because I didn't have enough of a build up of poly. I suppose I'll have to live with the plastic wood look, but I think it is better than leaving the wood to its own devices among the general public. I figure I can always sand the poly off down the road if it starts to fall apart for some reason.

dwhite

[Dan White]

Am I missing something on my final 2000 grit finish by

thanks for your advice.

dwhite

[J. Clarke]

However most people don't have the patience to flatten a surface with 2000 grit sandpaper. Sure, if it has a flat backing it will eventually get the surface flat, but you may die of old age before the job is done. The grit doesn't control flatness, but it does control cutting rate. The idea is that you start off with something that cuts fairly fast, get the surface flat quickly with that, and then use increasingly finger grits to take off the marks that the coarse grit made.

[George]

Regardless what you call it, if you have scratches in the surface, you won't get a tight droplet, and particles like those in coffee will hide from a swift wipe like troops in foxholes.

Now your experiment in using your surface as a hot plate is another matter. "Microscopic bubbles" is a red flag. You might be evolving excess moisture from beneath, and this will break your finish from the wood..

[Charles Spitzer]

thanks. what if it is already flat? doesn't poly, or other oil finishes, self-level pretty well?

[Mike Marlow]

Yes, but generally not smooth flat. There will typically be lots of inconsistencies in the surface. Everything from brush strokes if you use a brush to differences from the lap if you spray it, or even orange peel. Dust finds its way in sometimes and the less than perfect surface of the wood will telegraph in the finish. All of this - or any of this adds up to a less than flat surface. Depending upon how irregular it is, you will select a grit to take it down and then progress upwards in grit to be rid of the sanding marks. With sprayed finishes it's not uncommon to be able to start no coarser than 1000 grit. Typically, you can go to 1200 or 1500 from there and then to a fast rub with a rubbing compound for a mirror finish. In essence, flat is what you want to define flat to be. For some, nothing less than a mirror finish is flat. That absolutely calls for sanding.

[Dan White]

Hi. Yesterday I put another spill of coffee on the 2000 grit poly surface. This morning it was dried on. I wiped it off with a little rubbing and it came clean. However, by getting down close to the wood and looking across the surface I could see a very fine outline where the coffee stain was. It looks like maybe it etched the poly around the edge, but otherwise the finish looked the same inside and outside of the coffee mark. This outline is impossible to see and hard to feel unless you are really looking for it. I also put a hot coffee pot on the surface for 5 minutes or so to test the surface again. I got these very very small marks in the surface which could only be seen looking across the wood. I don't know how to describe them but I hit the surface briefly with some more 2000 grit and the coffee outline and these "bubbles" or marks did disappear. Maybe there is still some volatiles in the oil poly? It has only been a couple of days since I applied it.

I would consider coffee overnight or a hot coffee pot to be the most extreme treatment these counters are likely to see, and probably won't ever get the hot coffee pot. I was told that poly wasn't really impervious and that converted varnish was the only real way to get a bulletproof finish. It has to be sprayed on and I don't want to go that far.

It also seems to me that it isn't so hard to fix a poly surface if the damage is limited to the upper layers of poly. Why not sand it a little, add another thin layer of poly, and then sand it to finish?

thanks, dwhite

[Dan White]

I applied several (5?) coats of poly and then went right to the 2000. I'm left with a pretty shiny surface that has some ripples in it that cannot be felt, but can be seen if looking at reflected light at a shallow angle. It is not strictly a mirror finish but it does feel like one to the touch. I'm still not sure if a super shiny surface or a more matted one.

dwhite

[Mike Marlow]

That ripple is what you're looking to sand out when you go for flat. You can feel them if you run your fingers very lightly down the surface slowly. Over time you develop a good feel for finding them. Do a little autobody repair and you'll quickly become an expert at finding them. Either you do, or the word "dambkit" becomes a frequent part of your vocabulary - usually just after the clear coat goes on.

Grab yourself a paint stick and wrap it with your sandpaper (wrap the whole stick) and lay it flat on your surface and sand. It will flatten those ripples out nicely. Don't stop until the entire surface of the piece shows sanding marks evenly. Get it back up to the desired level of shine after that and then stand back and look at it at that shallow angle. You'll love what you see. For the type of ripple you're describing I probably would not go more aggressive than 1000 grit. You could go to 600 but you'd want to be quite careful. 600 can take finish down pretty quickly. 1000 will take more work but it will get there and there's less chance of burning through if you're not really accustomed to this stuff. Either way it's work. If you start at 600 your going to go through more stages to be rid of the sanding marks. If you start at 1000 you're going to sand more to get to the same level, but you'll only have one step before buffing. If you're really into it you can buff from 1000 but I prefer to sand more and buff less. I just hate buffing.

[George]

I'd bet on uncured vehicle, that's for sure. Week or more for full cure.

For repairs You'd have to do the entire, or see the edge. You might blend it flat, but it'll show a shadow.