Sand Between Coats Make sure to sand the polyurethane between coats. Sanding removes drips, bubbles and other uneven spots in each coat.
You are correct that, with polyurethane (brushed or wiped), you can recoat without sanding -- as long as you recoat within about 24 hours. After the poly has cured (usually about 3-7 days, depending on the product), you then have to sand. This abrades the surface and allows a mechanical adhesion between the two coats.
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.
If you are using a water based finish, such as the poly you mention, you would not need to 'tack' the surface between coats with a traditional tack cloth. A clean cloth dampened with water would be sufficient to wipe away any residual dust.
There are several ways to determine whether a surface is dry: OIL BASED products are dry if they are no longer tacky and do not emit a smell. WATER BASED products are dry if they no longer feel cool to the touch and a powder will form with light sanding.
Allow each coat to dry fully. To give the subsequent poly layers something to bond to, sand lightly between coats with 320-grit sandpaper wrapped around a hard block. Note: The first coat needs the most sanding to appear smooth; don't worry if it doesn't look as flawless as you'd like at first.
Polyurethane will dry to touch in 3 hours. Under ideal drying conditions, allow 6 hours before recoating. Overnight drying is recommended.
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.
I use new or just-washed microfiber cloths to clean camera lenses and computer screens, then they go into the pile I use as "tack cloths." Microfiber cloths can be washed, but great care must be taken to use detergents free of perfumes, oils, or anything else.
Two coats of finish are suggested for most interior projects. When sealing floors, you should apply three coats. However, do not apply more than two coats in one day. 220 grit sandpaper should be used between coats to remove any air bubbles, but avoid using steel wool.
It's all about adhesion. When you recoat after 2 hours you get a chemical bond between layers. If you let it go longer than that, you need to wait 24 hours so it's hard enough to sand and get a mechanical bond.
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.
Acetone is far from the only option available if you want to remove polyurethane. Many people use sanding strips, sanders, or metal scrapers to gradually remove the substance. You might also opt to use a planer, which trims the surface to expose the bare wood underneath. Chemical strippers are another popular option.
Sanding Tips: No sanding is needed if you overcoat within 48 hours. Don't sand unless the most recent coat has dried for 24 hours. Use 320 grit to prevent scratches; sandpaper finer than 320 grit will not allow proper adhesion. Remove residue before reapplying.
Use Between Coats – After each coat of paint or stain, let the surface dry completely. Use your tack cloth between coats to ensure a smooth finish. Store or Dispose of Your Tack Cloth – If your tack cloth is still usable, store it in a sealed jar or plastic bag to keep it sticky.
Microfiber cloths not only remove dust and particles from sanded surfaces very effectively but, unlike tack cloths, they can be washed and reused over and over again, making them economical and environmentally friendly. There is no perceivable difference from traditional tack cloths in how microfiber cloths are used.
Tim Inman: Woodworking and wood finishing textbooks all offer recipes for homemade "tack" cloths. Usually, they use some varnish or oil such as boiled linseed or tung diluted with mineral spirits to moisten the lint-free cloth. Actually, any oily substance compatible with your topcoats and stains would be fine.
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.
Use a Roller on Large, Flat Surfaces
rollers lets you apply the poly fast and evenly. No brushstrokes, puddles or thin spots.
When it comes to hardwood floor polyurethane, lower humidity equals faster polyurethane dry time. So, one of the best solutions can be to use dehumidifiers to speed things along. Also, be sure to clean your air conditioner's filter before starting your project.
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.