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Is there any way to prevent bubbles when applying polyurethane on old wood (it had paint on before but got removed, if that matters)? I just applied the first coat, will it be better after I lightly sand this coat with fine sandpaper and then apply a second?

I am using Minwax Fast Drying Poly Semi-Gloss, not because I needed fast drying but because that's all they had at Lowes. Could the fast drying property be causing the bubbles. Using single use sponge brush and 4" roller.

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    Bubbles are less of a problem than rough texture due to overworking. Apply heavily, then brush off until you leave a thin film. Move on. Don't work it longer than 2-3 minutes. Don't work in hot weather or in the sun.
    – isherwood
    Jul 16, 2021 at 13:24

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Polyurethane and other varnish type coatings are usually installed with a very good fine bristle brush. Brush only one way and use as little brush movement as possible. A back and forth motion will enhance the bubbling that you are experiencing. I do not believe that you can get a good finish using a sponge brush or roller and make sure that the surface to be coated is free of all dust. I always use a tack cloth to remove any and all surface dust before applying the finish. Years ago, when all finishes were of an oil base we would also warm the polyurethane or varnish by putting the container in a bucket with hot water. To get a good finish you need to use only the best tools. My 2 cents.

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  • This is absolutely correct, especially "you need to use only the best tools", which a foam brush is NOT. And for what it's worth, and it may seem silly to say it, never shake poly cans, stir gently only.
    – Jimmy Fix-it
    Jul 16, 2021 at 14:10
  • I find that neither thinner nor mineral spirits are adequate to clean/restore quality bristle brushes after use. If a quality brush is $7-8+, that means $7-8+ per coat of poly, as opposed to ~$1 with foam. Is there an effective way to clean poly? Thinner works fine with stain but not poly.
    – amphibient
    Jul 16, 2021 at 16:37
  • @Jimmy Fix-it; I forgot about the "never shake poly cans, stir gently only" item. Thanks for your input and my memory loss.
    – d.george
    Jul 18, 2021 at 9:57
  • @amphibient, water-based poly cleans out of brushes with soap and water. Oil-based with mineral spirits/paint thinner, turpentine, or even diesel or kerosene. I thoroughly clean and reuse my quality brushes many times over and use them for years. Maybe your cleaning technique is not the same as mine, which is a whole other topic...
    – Jimmy Fix-it
    Jul 18, 2021 at 19:38

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