Using a polyurethane can be simple, but when you have bubbles show up it can be frustrating, especially if you don't know what caused them. The following tips will save you time and frustration before you begin your next polyurethane project.
Things You'll Need
- Oil-based polyurethane or water-based polyurethane
- Bristle natural brush for oil-based polyurethane
- Nylon brush for water-based polyurethane
- Sanding sealer
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Use a nylon brush when working with a water-based polyurethane and a natural bristle brush for oil-based polyurethane. Because the texture and consistency of the two formulas are different, it makes a huge difference in the brush you use with each. The wrong brush can cause the poly to adhere to the wood differently, thus causing bubbles or streaks. It is also important NOT to over brush.
Try using thinner coats of poly. If the poly is too thick, it will take longer to dry and surrounding elements may affect how the poly dries, such as air flow and ventilation. Though you need to keep the room you are in well-ventialted, if it is windy, keep the piece away from the windows or open doorways. Keep free-standing fans away from a piece that has just been finished with poly. The strong current of air can cause shifts in the poly making it look like it has bubbles or waves. Having the poly too thick can also cause a problem with the top drying faster than what is underneath. Bubbles will often form as the underneath poly is trying to breathe and dry under the hardened poly. More thin coats is better.
Clean all sawdust and other dust particles from the surface of the piece that is to be finished. Often if you poly over the top of sawdust or other particles that did not get wiped off properly, it will cause bumps that look like bubbles.
Try using a sanding sealer before the first coat of polyurethane. Each wood is different and so are the pores and textures of that wood. It may be that you just need to seal off those pores so that the poly goes smoothly on. Sanding sealers can be purchased at all paint stores and most home improvement stores.