Grit of sandpaper to use between coats of poly?

Finishing the desktop I've mentioned elsewhere. I'm using very pedestrian stuff: MinWax Poly.
I made a test piece using the three types of wood that make up the desktop. For the test piece, I sanded to 220, wiped with mineral spirits, and applied Semi Gloss poly. I then put on two coats of Satin Poly, lightly machine sanding with 220 in-between. I liked the results, except for some run marks that probably resulted from too thick a coat plus hanging the test piece vertically.
Why two different glosses? A friend of mine did something similar on some flooring with good results, plus I had some semi laying around. I don't know if using semi for the first coat had any practical effect, but I decided it couldn't hurt.
I've now got two coats on the actual project. Not content to leave well enough alone, I varied the procedure just a little bit. I again sanded to 220 and applied semi gloss first. That was Saturday. I had a (music) gig Sunday afternoon, so I got up early to see if I could get in another coat. Our house is attached, and thought the power tools at 7:30 in the morning might not be appreciated next door, so I decided to try hand-sanding at least the edges.
I used 400 grit, which I had never tried before. As the edges are rounded, I used it without a block, just with fingers. It gave the edges (oak) a lovely smooth feel. I decided to do the whole surface with the 400, using a block for the large area. I then applied another coat of satin.
I took a look at the result after my gig. It's coming along. I decided not to take on putting on another coat tonight, but I couldn't resist sanding it a tad, to see what the feel would be like.
So here we're coming to the question, finally. I again used a small piece of sandpaper in my fingers for the edges, but decided to try 600 grit. I figured if it didn't adequately knock down the small amount of roughness that remained, I could go beck to the 400.
I was surprised by the result. It smoothed the surface nicely, but didn't appreciably dull it. It looks and feels quite nice, even though I plan to add at least one more (and likely another) coat.
I used the same 600 grit on the large surface area, using a block with a piece of scotch-brite in-between the block and the paper to "soften" the block. I don't know if this made any sense at all, but it didn't seem to hurt anything. This surface did not start out as smooth as the oak border, and the sanding did give it the slightest "white" cast (dust) when viewed at the right angle against the light.
I'm wondering if using the 600 paper is likely to bite me in the ass in any way later. Does the next coat of poly need more roughness to adhere? Am I giving up the chance to more thoroughly smooth the main area?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

220 between coats is fine enough and gets the job done fast. If you want a gloss finish, use lacquer or shellac. Either one is harder than poly and rubs out better.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 16 Oct 2011 21:08:03 -0700 (PDT), Father Haskell
nother) coat.

piece of scotch-brite in-between the block and the paper to "soften" the block. I don't know if this made any sense at all, but it didn't seem to hurt anything. This surface did not start out as smooth as the oak border, and the sanding did give it the slightest "white" cast (dust) when viewed at the right angle against the light.

way later. Does the next coat of poly need more roughness to adhere? Am I giving up the chance to more thoroughly smooth the main area?

If you use 220 between coats, put on four coats, then let it cure for a few weeks. Now you can polish it out with 320, 600 wet, rottenstone, pumice, and finally, a coat of wax. It will give a really nice professional looking finish.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Your work will pay off a lot more with lacquer or shellac. They're not "difficult" to apply if you lay them on fast and thin -- trying to fill in skips before the coat is dry is what causes ridges. Actually, they're a bit less work than poly, since you don't have to sand between coats.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Greg Guarino wrote:

Sanding varnish between coats does two things: 1. removes nibs/runs 2. abrades the surface so the next coat has something to stick to A grit of 220-240 is plenty fine for that, 600 & 400 are pointless unless it is necessary to even up the surface before final polishing.
It isn't necessary to sand for adhesion *IF* you recoat within a particular time interval...4-8 hours IIRC, tells you on the can. I generally put on 3-4 coats (not wipe on), all the same day. ________________
All clear coats begin life as glossy. That sheen is modified by adding a flatting agent. The more flatting agent, the less sheen. Those agents not only decrease sheen, they decrease - very slightly- transparency; therefore, some people (not including me) use only glossy to build and a less glossy one for the final coat.
As you discovered, different grits give different sheens. Grits are available up to at least 12,000. For final finishing one is better off using a flexible pad; in the case of pumice/rotten stone, a felt pad; easier is FFFF steel wool followed by a coat or two of paste wax...that won't give you a high gloss like pumice/rotten stone but it *IS* easy and gives a nice sheen. __________________
Father Haskell said that shellac and lacquer are harder than poly and rub out better. I agree with the last, not with the first.
The problem is, what constitutes "hard"? To me, it implies resistance to deformation and that implies inflexibility. Like glass. One wants some flexibility in a clear coating. A spar varnish is flexible but one also wants scratch resistance and spar varnish doesn't have that. AFAIK, the most scratch resistant, one part clear finish is oil base poly non-spar varnish.
--

dadiOH
____________________________
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 10/16/2011 8:51 PM, Greg Guarino wrote: . It looks and feels quite nice, even though I plan to add at least one more (and likely another) coat.

of scotch-brite in-between the block and the paper to "soften" the block. I don't know if this made any sense at all, but it didn't seem to hurt anything. This surface did not start out as smooth as the oak border, and the sanding did give it the slightest "white" cast (dust) when viewed at the right angle against the light.

way later. Does the next coat of poly need more roughness to adhere? Am I giving up the chance to more thoroughly smooth the main area? If you switch to a gel varnish you no longer need to do any sanding/scuffing between coats and you end up with a smooth satin finish.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Site Timeline

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.