Yes, it is possible to apply a new coat of polyurethane over an existing coat without sanding, but it's not ideal. Sanding the surface before applying a new coat of polyurethane is important for proper adhesion.
Make sure to sand the polyurethane between coats. Sanding removes drips, bubbles and other uneven spots in each coat. Dry the first coat for a full 24 hours. If the coat feels tacky or sticky after that, let dry until it feels completely dry.
Most instructions will say something like "recoat in 2 to 6 hours, or lightly sand to recoat". That first window is the chemical bond, and after that is mechanical. You need to scuff sand to ensure a good bond. Simply lightly hand sand with 400 grit or similar.
Waterborne polyurethanes usually require a minimum of 3-4 coats to be effective. Although there is some evidence that too many coats can result in a plastic-like finish, waterborne polyesters are thinner than other types and therefore require several more coats to provide the same protection for hardwood floors.
Polyurethane is a reactive coating - it will not re-dissolve once it is cured. Any coating that is applied on cured poly is a gamble, and in most cases will fail. Naturally, the best way to go is to strip it, and refinish it.
Prepare the polyurethane by reducing it about 10 percent with mineral spirits, unless you're using water-based polyurethane, which doesn't need thinning. Thinning the polyurethane makes it flow on more smoothly and reduces brush marks. Quick Tip!
Avoiding Sanding and Priming Prep
Surfaces with varnish, polyurethanes, or other sealants or finishes require sanding before any paint can be properly applied.
Yes, it is possible to apply a new coat of polyurethane over an existing coat without sanding, but it's not ideal. Sanding the surface before applying a new coat of polyurethane is important for proper adhesion.
As a general rule, polyurethane can take as long as 30 days to completely and correctly cure. It would be best to give yourself a full 3- days before using your wood floors following a polyurethane treatment for the best possible outcomes.
You should use an automotive polishing compound and a buffer for big areas. Small areas will have to be polished by hand unless you have some funky buffing tools. Make sure the poly has cured for a minimum of 4-5 days at 70f. Do not try to polish an uncured film.
Water-based polyurethane requires at least two hours of dry time between coats, and you should only apply two coats in a day. It's important to remember that drying time is affected by temperature and humidity.
Do You Have to Sand Before Staining Wood? Wood needs to be sanded well before it can be stained. The process opens up the wood's pores to better absorb the stain. It also smoothes away scratches in the surface which will only be enhanced by stain.
A finish that remains tacky long after the expected drying time may be due to poor preparation, inadequate drying time between coats, or high humidity. To address this: Allow more drying time in a well-ventilated area. If tackiness persists, lightly scrub the surface with steel wool.
Sand it with 220 on a foam block, vacuum off, sand with 400, vacuum, 0000 steel wool, vacuum, tack rag, and apply your final coat with a wide foam brush. Before my "final coat", even if my previous coat was perfect, I sand with 400-600 and 0000 steel wool to make it silky smooth before that last coat.
Three coats are recommended. After final coat, allow 3 hours before light handling and 24 hours before normal use.
The mechanical bond created by the sanding will help the layers adhere more securely, reducing the risk of delamination. If you're applying a final topcoat or if the subsequent layer is the last one, sanding can also help remove any imperfections or bumps from the previous layer, ensuring a smooth, high-quality finish.
There are two ways to tell if it is an oil-based product is dry. If it is no longer tacky and it doesn't smell, it is dry. You can tell if a water-based product is dry by touching it. If its cool to the touch, it's not dry.
Humidity and Temperature Effects: Environmental conditions play a pivotal role. High humidity coupled with lower temperatures extends drying durations, while low humidity and higher temperatures expedite the process significantly.
Apply 4 coats of oil based polyurethane on wood and wait 8 hours in between. Don't apply too many coat and you should be just fine.
To ensure a smooth coat that is relatively free of brush marks, use a good quality, natural-bristle brush. I've gotten great results with a brush designed for oil paint available at home centers. Brush on a thin but wet coat, quickly smoothing out any runs or drips as you go.
One evenly spread coat will make your furniture water resistant. Most furniture builders, however, will put on three coats, with a very light sanding using 0000 grit steel wool, then using a tack cloth to remove any dust before applying next coat.
The Short Answer: All polyurethane will yellow over time. While modern technology has extended the amount of time it takes to yellow and reduced the extent it can yellow by, ALL polyurethane (including polyacrylics) will yellow.
Even though oil primer smells bad and can be messy to work with, it's the best option for painting over polyurethane because it soaks into wood to form a seal without raising the grain like a latex primer would.
Understanding Deglossers: What is a Deglosser? Deglossers, often referred to as liquid sandpaper, are chemical solutions designed to remove the glossy finish from surfaces without the need for physical sanding.
The short answer is yes, to a certain degree. There are several factors that make some polyurethanes more absorbent than others. Depending on the physical properties and material, thermoset polyurethanes can practically have zero water absorption compared to other known materials.