How to get smooth urethane finish?
 

I made some shelves. Nothing fancy, Baltic birch ply with a cherry 1x2 facing on the exposed edge. I wanted to urethane it to stand up to kids, wet, etc. So, I get a can of "spar urethane" because the lumber store guy says it has a bit of amber hue. Can says "no brush strokes". I sand the wood with 200 grit, clean off the dust, brush on according to directions, three coats, dry 6+ hours between, sand smooth after the second. Well, the finish is not smooth, it is sort of lumpy or stipply, not brushstrokes but little bumps. What am I doing wrong. I can sand the finish smooth again and re-apply but I think I'm missing something. The last time I used varathane with the same process and it came out very smooth, so I thought I knew what I was doing.

Also, when I sand it smooth for another try, should I use something else, finer grit or wet sanding?

Not exactly the same application but on my boat I use the cheap foam applicators to apply varnish, no brush strokes.

What environment are you doing the finishing in? Sounds possibly like dust particles are settling on the wet varnish.

200 grit is not enough, sand progressively up until about 800, maybe more. I like wet sanding.

It has been an awful long time, but I always sanded with steel wool before and then between the coats. You need to use a tack cloth to make sure you get the dust and the steel wool off, after each sanding.

Like redbeard stated, a tack cloth is needed. When I refinished my oak floors, I was very careful in using tack cloths and it turned out smoother than a baby's butt. It's all in the prep!;)

Vacuum then tack cloth the surface. Personally I like the Varathane spray. Goes on nice.

Did you seal the grain of the wood first? before applying the poly?

While birch doesn't have an open grain like mahogany, a sealer can really help give an even surface for the top coats..

http://hardwareaisle.thisoldhouse.com/2008/01/sanding-sealer.html


Though that is moot now since you already have a few coats on.. fine sandpaper and tack rags now..

Okay, sounds like I was not careful enough. I sanded the bare wood after wetting it, thought I had gotten the fibers down. But I must have been sloppy somewhere in the process. Okay, will re-sand then wet sand with finer grit and tack. Thank you.

When you wet dry wood (with whatever) there is some sponge action. The softer the wood the more sponge action you get. This will raise imperfections and cause the stipply look you're getting.

You need to first seal, which in this case will be your first few coats of polyU and then sand. 200 grit is not fine enough at all. You need to get to at least 800 maybe 1000 if you're really into it.

I don't like this spar urethane much. The last coat was over 24 hours ago and it is still a little bit tacky. I think I will go back to varathane for the re-try.

Stop mixing the various finishes

Again moot since you already applied spar over the other finish...

Sometimes its better to let coatings cure longer than the minimum re-coat window.. You can take the piece outside and get some UV curing other than chemical or evaporative curing....

Though you mentioned you waited six hours for re-coats...

I'd wait a few days and let all the finishes you applied fully cure and see which path to take after that.

Before you brush it on, make sure it's stirred, not shaken.

Jyl,

I like to thin the first coat because it dries a bit faster and sands easier. Do sand between coats. Do not, do not shake the can. stir only. that can cause little tiny bubbles in the poly and it does not go away when you apply the finish with a brush. You have to wait for days for it to go away on its own. I do not usually like to back brush or press hard as it can cause bubbles as well. Try going one direction with the tip of the bruch and don't over apply the poly. It should go on smooth after the 2nd coat.

I like to apply 2 coat base coat, sand between coats, and spray the final coat, even if its rattle can. It does not go on too bad at all.

If you want it to dry faster, add a little of Japan Dryer to the poly. That should speed thing up a bit. Good luck.

Jeff

Have you tried using a mini roller to apply? Since these came out, they are my favorite painting implement. You would need a lint free roller, possibly a sponge. These are ideal for doing flat surfaces like doors and shelves. Much faster then a brush and no stroke marks. Just need to keep a wet edge and don't back roll once your done.