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My last polyurethane finishing experiment failed. I applied fast-drying Varathane to 3/4" maple plywood shelves. They were very finely finished before applying the polyurethane, and became very rough after. I suspect that the culprit is a combination of overzealously going over surfaces more than once and using a fast-drying formula.

It's almost springtime and I'm now about to apply polyurethane to floors (after sanding them). I've learned the two lessons (use a slow drying formula to forgive minor overcoats, and attempt not to overcoat in the first place). Of course I'll be sure to remove the last speck of wood dust before finishing.

The last potential issue is the brush. I've used a general purpose brush (similar to those one might use for painting walls). But the first comment at the link above suggests using a painting pad / sponge squares. My local store's website lists "paint edger", brushes , and of course rolls. What are painting pad / sponge squares called at the main chains' websites, and are these indeed the simplest way to get a smooth finish after one application? My objective is to apply one layer this year and repeat when necessary, since I see that even the pros do not bother with two layers so I'd also rather not.

  • Going over the poly will make brush strokes in both water based and oil based. Get your coat down and let it dry. If you want a glass smooth finish 600 grit lightly sanded and cleaned between coats will work well. – Ed Beal Mar 17 '16 at 18:26
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I have applied polyurethane with foam brushes many times. I refinish wood infrequently enough that cleaning brushes is not worth it to me. In my opinion they work great. The trick is to reduce adding to much air to the finish:

  • gently stir the can of polyurethane before you use it but do not shake like paint. Shaking will add a huge amount of air (bubbles). You can pour the finish into another cup for dipping into or just brush straight from the can.
  • dip the foam brush into the polyurethane just enough to partially load it. you do not want the whole thing full of polyurethane and then make a big puddle when you touch the wood.
  • DO NOT SCRAPE THE FOAM BRUSH ON THE EDGE OF THE CAN. just hold the brush above the can and let any extra finish drip off. You can wiggle it a bit but if you scrape the foam you will add a ton of air.
  • As you said, don't go over the finish again. If you end up missing a spot you will get it again on the next coat.
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That sounds like water based polyurethane. Water based polyurethane is more difficult than oil based poly. You can recoat water-based poly with oil-based.

If that sound like something you want to do, then the first thing to do is let your shelves cure for 2 months. Then use fine (150 then 220 grit) sand paper. Clean the shelves thoroughly; dust with a cloth dampened with mineral spirits.

Ventilate the area well and wear a respirator rated for organic fumes (follow the label warnings on the polyurethane can) when applying the finish.

Sponges and brushes are just different. Either one takes getting used to. Personally, I often use a lint free cloth over a sponge and I replace the cloth often. I am not saying that my way is any easier than another. Only practice makes perfect.

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Painter's pads for varnish are a compromise between the traditional approaches of applying with a cloth pad (thin coats so more needed, but they go on smoother and dry faster) versus applying with a brush (thicker coats, slower drying of each, more care needed to make sure you don't leave brush marks). So far in my limited experience I find I'm happiest with the results from a cloth pad, but this is a matter of picking what works for you on this sort of project; it's all tradeoffs.

By the way, some commercial varnishes labelled as brush on work just fine padded, others do so if thinned slightly ...and either option tends to be cheaper than buying varnish sold explicitly for pad application.

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