Polyurethane wierdness

[Rob Blaustein]

Question: if you don't stir a can of satin water-based poly finish, and just brush on the stuff that's at the top of a new can, are you effectively applying a gloss finish? In other words, does the stuff that's added (silica etc) to make it satin settle to the bottom?

I ask this because of some strange results I've gotten recently while experimenting with finishes. I applied 2 coats of Zinsser SealCoat (2 lb cut of dewaxed shellac) to some birch and walnut veneered ply, followed by 2 coats of General Finishes High Performance water-based poly in satin finish, applied with a foam brush. In both cases, after the second coat I got a nice hard and smooth shiny finish that felt almost glasslike. I thought it looked more glossy than satin, but I thought it would be fine for the inside of a vanity, so went on to finish the actual panels. On the advice of some here I only put on one coat of shellac, then put on two coats of the poly. But the finish was very different. Not as smooth, and not nearly as shiny. Put on another coat of poly--no change. I thought maybe it was because of one vs two coats of shellac, so I tried putting two coats of shellac on a test piece, followed by 3 coats of poly. Same thing--a satin finish (as advertised), not as smooth or shiny as the original test pieces. Then it occurred to me--when I first used the poly I forgot to stir it. Before applying it to the actual panels of the vanity I stirred it up well, noting that the stuff on the bottom was much thicker. So if my hypothesis is correct, is it also true that a glossy finish is smoother in addition to being shinier?

Another question: if you want a finish to be glossier and you've already applied satin coats first, can you just apply a coat of gloss or semi-gloss and get that effect? I don't have any gloss or semi gloss around to try, so I thought I'd ask.

[Steve Schoene]

Yes, the flatting agent does settle out. And, the way in which it works to make a satin finish is by providing some extra "roughness" in the surface. You would have somewhat similar extra roughness if the satin finish were created by abrading the surface with pumice.

And, since you used some of the glossy varnish off the top of the can with the first experiments, when you did stir up the can you likely got a flatter finish than the makers had intended. Also remember to keep stirring the finish frequently while you are applying it to keep the finish from being streaky.

[Rob Blaustein]

Thanks Steve. Any input on applying a gloss over satin? If I wanted to "polish" up or smooth out the satin finish a bit, is it reasonable to lightly sand with 600 and higher?

[Steve Schoene]

If you want to apply another coat of finish over the satin you can use 320 grit. Gloss will go over satin, but the overall clarity of the finish will be less than if it were all gloss, because of the flattening agent suspended in the finish. But mechanically there would be no problem.

[Ken Fitzgerald]

Rob......normally I think they recommend using a gloss and then lightly sanding it with steel wool to take it down to a satin finish. I don't think you can buff a satin up to a gloss. And as Steve said, adding a gloss over the satin won't produce a real gloss finish....

[Rob Blaustein]

Thanks guys. So maybe if I put on some gloss over the satin I'd end up with semi-gloss, but smoother feel? Well, sounds like another trip to Woodcraft is in order, followed by some more experimentation.

[Brandon Shew]

Gloss over satin will not make it more glossy. You need to get rid of the satin by sanding it off and then re-applying gloss to get more of a gloss finish. A glossy finish is not necessarily smoother - that depends on how its applied. (I'm also not a huge fan of water based polys.)

When using Poly I don't see any need to apply it over shellac. Unless the shellac has some coloring agent added to it, I would just go w/ poly straight on the material.

Here's the (oil based) poly process I use. It's a little tedious, but it produces superb results:

1) Use foam brush and apply 1st gloss coat and let dry. This coat will mostly soak into the wood. and server as a base or primer layer to seal the wood.

2) Dry sand w/ 220 grit until the surfacfe is butter smooth. You may end up taking it back down to bare wood - or really close. Clean w/ a tack cloth or damp rag (dampen w/ mineral spirits).

3) Apply 2nd coat of gloss w/ foam brush. This helps to fill in most pores and low spots that are left.

4) Wet sand (I usually use mineral spirits for the wet part) w/ 400 grit until it's really smooth.

5) Repeat steps 3 & 4 at least 1-2 more times or more if using a really porous wood. The last wet sanding I do w/ 600 grit.

6) Here's where you determine the gloss factor -
A - for high gloss - lay on a final coat of 50% gloss poly & 50% mineral spirits w/ a brush. The mineral spirits help it flow better, eliminate the brush strokes & dry quicker.
B - For semi-gloss wipe on the 50/50 mix above w/ a lint free cloth
C - for a matte luster, wipe on a 50/50 mix of satin poly (shaken or stirred well) and mineral spirits.

Follow that procedure w/ good sanding between each coat and it will make one of the best Poly finishes you've ever seen.

[Steve Schoene]

Let me comment a bit more. With gloss over sanded satin you will still get a gloss finish, just not as totally clear. The difference will be minor.

Between coats, you don't need to go finer than 320. The 320 will give a good tooth for bonding the subsequent coats. Too fine and adhesion could suffer, though 400 grit won't be a disaster at all, its just overkill.

With any varnish I believe you must rub out the final coat, since I can't imagine avoiding enough dust to skip it. To accomplish a good rub out the finish must be flat and even, with no chance of cutting through the final coat during the rubbing out process. I think there are two things that help this happen. First the surface must be truely smooth--that's where the sanding between coats is important, just as Brandon suggests. Its the second step where I differ a little. I wouldn't thin the final coat so heavily since I am enough of a klutze as to cut through a very thin coating. Of course, all varnish coats must be thinned, especially since VOC regulations mean that the manufacturers don't add enough thinner at the factory. And, they lie on the label about not thinning--since if they told you to add thinner the regulators would say that was roughly the same as adding it themselves. This I would thin the final coat just enough for it to flow out smoothly. This will usually be accomplished with 10% thinner, give or take.

The very thin coats do tack up quicker since they are so much thinner. I suppose if you have a very clean environment this could be fast enough to miss most of the dust, but I've never really been able to get that clean a place to varnish.

Finally, for the best results, I would recommend staying away from Poly. Poly is much harder to rub out properly. It is too soft. A good traditional resin varnish works much better. Behlen's Rockhard on darker woods, and Pratt & Lambert 38, or McCloskey's Heirloom on lighter woods will all work. For really high gloss go with lacquer or shellac.

[Rob Blaustein]

Thanks Brandon and Steve. Steve, I don't know if this applies to all water-based poly, but I've read that they are harder than oil-based poly finishes which may help rubbing it out. At some point I'll try the Behlen's Rockhard--people here seem to like it.