How to apply water based Polyurethane without a brush is one of the quick answers.
If you brush along the grain, you can stretch the polyurethane into a thin coat.
If you want to give the surface time to cure and harden, allow it to dry for 24 to 48 hours.
Start with a thick coating sand and let the last coat dry for at least 24 hours.This is standard practice for any wood finishing job.
Don't apply more than one thin coat.Then, use a piece of brown paper bag or 1,500-grit sandpaper to remove the dust.The photo below shows how to buff the finish to a high shine using automotive paste wax and fine abrasives.
In the range of -62C to 93 C, polyurethane can be used.It is possible to extend the performance reach of polyurethane to as high as 150C.
The area is covered with a light coating of finish after sanding.To remove dirt from the sanded area, brush it off with a soft cloth.Make sure to wipe off any extra items after applying the finish lightly.
Keeping this in view, what happens if you don't sand between coats?You have to apply each coat of finish within a reasonable amount of time.Sanding will smooth the finish and your next coat will look better if you work on a smooth surface.
Shake the can and improper brush prep are the most common causes of bubbles.
You can use a paint roller.It is easy to apply Polyurethane with a paint brush or roller.Applying the material with a roller eliminates brush strokes and lines.
It doesn't do much good to have more than 3 coats of poly.It is not needed or recommended.You have to buff off half of the previous layer for each additional coat.3.5 coats is more like 4 coats.
If you want to change the sheen from high gloss to matt, then using a hardening oil such as tung oil is a good way to do it.Let the base coat dry and then use 220-grit wet/dry sandpaper to smooth it over.Apply a second coat.
You don't have to sand the final coat.Let it dry before applying the final coat.When you sand and rub out the final finish, you take off most of the surface.
The bristle or foam brush is about 25′′ wide.The foam works well because the brushes are disposable and cheap.You can apply the first coat full strength or thinned up to half with paint thinner and make a wiping varnish.You can use a separate can or jar.
Give your finish a good buffing with a new screen.There are flaws on the finish such as bubbles, raised grain, and brush marks.Uneven sheen and coating are fixed by applying a more even second coat.