Polyurethane problem

[Brian Siano]

I'm refinishing a chest of drawers, and the last part of the job is the top. I've sanded it nice and smooth with 320-grit sandpaper, applied a layer of poly, let it dry, abraded it and applied a second layer... and now the poly's gotten, well, not so smooth. It's a little warpy, with a pronounced brush stroke texture.

I just tried sanding it down again, with 100 grit, then 220 grit, and

320 grit-- but I couldn't seem to sand it perfectly flat now. Even tried wet-sanding. I've laid down another layer of poly in the hopes that it'll even out. But I'm not optimistic.

So... any suggestions?

[ejb]

Hi Brian,

Applying poly or varnish by brush is a skill that takes a lot of practice!

Most poly finishes are "self leveling". This means that the brush marks and lap marks flow out very shortly after you brush on the finish leaving you with a flat surface. There are a few things that can prevent this:

1. Over brushing. Here's the rule: never go over your brush strokes. Use one or two brush strokes to get the finish onto the surface and that's it. Never go back and try to brush finish that's already on the surface. The overlap between sections should always be "feathered" in. This is done by slowly lifting the brush as you finish the stroke. Always work with a wet edge. This means working quickly!

1. The finish is drying too quickly. If it starts to set up before it can "self level" then you'll see the brush and lap marks. Read the label and adhere to the environmental specs (temperature and humidity). Or, if you can't control the environment, you can generally thin the finish (or get a retardant) to slow the drying rate.

2. You'll never ever brush the perfect surface (at least I haven't!). Expect to apply several coats so that you build up a surface that can be wet sanded and hand rubbed after it's all dry. This is how you get that perfectly smooth, deep, rich finish. Even a spray on poly needs to be wet sanded and hand rubbed in the end.

There are some books on finishing which might help. I recall one from Taunton which was very helpful but I don't remember the name. If I were at home I could just look on the shelf. I think it was something like "Fine Woodworking on Finishing". Basically articles of one topic (finishing) from the magazine all bound into one book.

Let me know if you have any questions. Done this a time or two!

Ed Bennett [e-mail]

Home of the TS-Aligner

Brian Siano wrote:

[Gregory Paolini]

While I don't do much refinishing, I do apply lots, and lots of polyurethane - Here's some of my suggestions:

First, I'm not sure about the 100 grit sanding between coats, or was this an attempt to remove the previous poly? Either way, start by doing a general sanding, with about a 180 grit paper in a random orbit sander. This is to knock down the previous coats and dust nibs. Don't worry about removing all of the previous coats, you're just looking to smooth out this surface and make a little tooth on the surface for the subsequent coats to adhere to. Vacuum or blow off the surface.

I'm not sure if you're using waterbased or oilbased poly - Assuming it's oil based, thin the poly with mineral spirits - in about a 3 parts poly to 1 part spirits mix. Off the shelf poly is too thick to apply out of the can. Don't worry if the can says "Do not thin" - That is printed there so the VOC level stays low enough to sell in California. Next, Use a good brush, I usually use Purdy brushes, and dip the brush fully in mineral spirits - This will help the brush flow better.

Dip the brush 1/2 way into the poly mix, and pat it against the inside of the container - Do not scrape it along the edge - and using a full brush stroke, with the brush held at a 45 degree angle, glide along and off the end of the top.

Repeat, overlapping only enough to avoid dry spots.

Charge (Dip the brush into more finish) when you start to feel it drag

Do not fuss with the finish - Let it level itself out while it dries - The extra mineral sprits in the poly will aid in this.

After the poly dries, scuff (barley the weight of your hand) sand with 320 paper and remove the dust.

Apply about 2 more coats following this procedure, then allow to cure for 2 to 3 weeks, then rub out the finish with mineral oil, pumice, and 0000 steel wool

Hope this helps Greg

[Brian Siano]

That's a strong candidate for blame here. (Another factor was a recent rainstorm, and the humidity influx might've played havoc.)

Right now, I have a layer of 3M's safest stripper taking the polyurethane off, so I can do the top again from scratch.

The sides finished just loverly. Why does the -top have to be such a pain?

[bob]

1. If it's a can of poly that's been opened before and has been sitting around, it is quite likely that some of the spirits have flown, so to speak, and the poly has thickened some. If a can is not sealed very tight, this will happen. You can either thin it, or just get a fresh can. Poly is always a bit too thick for me, even when new, so I prefer to thin it before using it to avoid this kind of build-up.

[ejb]

I've noticed a couple of different symptoms with humid weather. First, the finish remains tacky or soft. It will eventually dry out but it might take a long time (i.e. weeks!). Don't recoat over a tacky surface, you'll only trap solvent in the previous coat and extend the drying time even longer.

Humidity can also cause a sprayed coat to "blush". Rapid evaporation of the atomized finish causes moisture to condense out of the air and get into the coat. The trapped water makes the surface look cloudy.

The sides might have flowed out better with gravity. But, it is more likely that you used a thinner coat, brushed it less, and just can't see it as well. Broad horizontal surfaces are subject to very close scrutiny. They demand a huge amount of careful attention.

Greg's message contained some great technique tips - some of the things I remember from my early reading on the subject. Unless you are committed to learning by trial and error, I think it would be wise to spend some time with the books before your next attempt.

Questions and comments are always welcome.

Ed Bennett [e-mail]

Home of the TS-Aligner