As a clear finish for woodwork, polyurethane has advantages over lacquer and alkyd varnish, but in addition, it has disadvantages. On the other hand, it’s a very hard plastic that protects well and can be relatively simple to apply. Negatives include the tendency to form bubbles that can harden in the surface and also a longer drying time than lacquer. Contrary to lacquer, polyurethane does not redissolve after it hardens, so issues should be resolved on a coat-by-coat basis. Besides bubbles, possible problems include checks, cracks, runs or separation and blushing. Many can be adjusted by sanding and recoating.
Pop bubbles that arise when painting or spraying polyurethane with the tip of a paintbrush. They are sometimes caused by the turbulence of the paintbrush or from moisture introduced by the substrate. It’s possible to keep them from forming in the first place by cleaning more gradually or, because off-gassing is much more pronounced on warm days, employing the finish at moderate temperatures.
Flatten bubbles that have hardened to the finish using 220-grit sandpaper and recoat themafter taking appropriate precautions to prevent them from reoccurring.
Sand a checked or cracked finish with 220-grit sandpaper and recoat it. These defects can seem if you distribute a thick coat on a hot day and it dries too quickly. After flattening the affected region with sandpaper, wait till the temperature drops and spread a thinner coat.
Wait for stirring to dry before you scratch them off using a razor blade or sand them down using 220-grit sandpaper and recoat. If you try to brush them out while the finish is tacky, you are going to produce more work for yourself, since you’ll leave brush marks that will also have to be cut out.
Scrape the finish from a horizontal surface using a pull scraper when it’s still moist and wash the surface with a lubricant if the finish starts to separate and form into beads. Use mineral spirits if employing a solvent-based merchandise and water to get a water-based one. This defect is caused by silicone from the substrate which might have been left by furniture wax or polish. While there is no simple way to remove silicone out of wood, you might be able to get most of it off by sanding. Insert a leveling agent into the foam to protect against the same thing from happening when you recoat.
Remove the finish with an oscillating tool, a sanding accessory and 100-grit sandpaper if it turns cloudy after it stinks. This defect, called blushing, is that the result of moisture from the wood and will be more likely occur when using a solvent-based product. It isn’t too common with polyurethane as it’s with lacquer, but might take place whether the wood you’re completing is very wet and the neighboring states are hot and humid. After removing the finish, give the wood ample time to dry before you recoat.