HELP !!! how to get a glass smooth finish with polyurethane

[Jim Colombo]

Water borne finishes...including the Polycrylic...will perform much better if sprayed. They dry so quickly that it can be a challenge for them to lie down, resulting in the bubbles, etc. For brushing, the actual brush is important, too...nothing serves you better than a very high quality brush if that is your preference for application. Oil based varnish is better, IMHO, for brushing or wiping because it dries slowly and can be more forgiving as it levels out.

I already have 2 coats of the water based poly on the tops. Could I still go to the oil based poly and put it on top of the coats already there?

Jim

[Jim Becker]

I already have 2 coats of the water based poly on the tops. Could I still go to the oil based poly and put it on top of the coats already there?

Jim

Shouldn't be an issue as long as you lightly abrade the finish already applied. It's best if it's had some time to cure a little, too. Water borne "poly" isn't anything like the oil based product. It's an acrylic that has some level of polyurethane resin in it. It also doesn't have the "warm" look of the oil based product. The light abrasion is to insure that the other finish will adhere to it....polyurethane finishes are not fond of sticking to themselves sometimes.

[Curt Harms]

Shouldn't be an issue as long as you lightly abrade the finish already applied. It's best if it's had some time to cure a little, too. Water borne "poly" isn't anything like the oil based product. It's an acrylic that has some level of polyurethane resin in it. It also doesn't have the "warm" look of the oil based product. The light abrasion is to insure that the other finish will adhere to it....polyurethane finishes are not fond of sticking to themselves sometimes.

Would shellac be appropriate as a barrier coat between water based and oil based? I've never tried shellac except spraying but it dries fast. I find that dewaxed shellac on raw wood under waterbornes works very well, there's no grain raising and amber or tinted shellac can add some color if desired. A light sanding and succeeding coats are smooth as a baby's butt. A little late for Jim but something to think about going forward.

[Jim Colombo]

Thanks for all the input but before I was able to implement anything, disaster struck. The panels (composed of laminated strips of poplar) both curled up on the sides (looks like boat bodies). I'm guessing it has something to do with the moisture in the 2 coats of poly already applied.

They curled up on the sides with the poly. Now I have to figure out if I can save them.

Any help in that area would be greatly appreciated.

Jim

[Bennett Ostroff]

If you haven't finished the bottom, you'll need to. That's likely the culprit of the cupping - uneven moisture absorption. Flip it over, put 2-3 coats on the bottom, and I suspect it'll be back to how it was in a day or two. Also, the thinner the panel, the more susceptible to cupping.

[Jim Becker]

Would shellac be appropriate as a barrier coat between water based and oil based? I've never tried shellac except spraying but it dries fast. I find that dewaxed shellac on raw wood under waterbornes works very well, there's no grain raising and amber or tinted shellac can add some color if desired. A light sanding and succeeding coats are smooth as a baby's butt. A little late for Jim but something to think about going forward.

Yes, De-waxed Shellac is often used as a barrier coat. I generally mention that, but I honestly just plain forgot. Thanks for the reminder!