As you see I got this rough finish at a wall after third coat. What is the reason that this happens? I am using wool 1cm ~ 3/8 roller. Also sometimes I get lots of tiny bubbles that do not go away no matter how ligthly I press the roller. I am using a satin water based paint. I am not sure if the product is bad since I have various issues like foaming.
Your roller seems very small for painting a wall. 1cm - 3/8 would work well for touching-up. Perhaps you should try a 2-3 inch roller, this would be much quicker, be more consistent and you’d be using the same tool as a professional would use. Additionally, if the paint you’ve used is old or been allowed to lose moisture it may be thicker than when new. As your paint is water-based, adding a small amount of water may help (try on a small area first). You also mention about bubbles and being ‘light with the roller’. Emulsioning can create tiny bubbles as the roller passes over the surface but theses will quickly disappear. As for being ‘light’, there shouldn’t be a requirement to be light with the roller to get a good finish. Buy a quality shirt-pile mohair roller for silk emulsion and a medium-pile sheepskin roller for Matt emulsion, avoid spongy foam rollers and you’ll be painting like a professional!
Rushing to apply a second coat before the first has thoroughly dried will create this heavy stippled texture.
When paint is only dry to the touch a second coat reactivates the first. This effectively reduces the available moisture needed to flow the paint by 50%.
I had a similar problem when I tried to put a third coat on too soon in my living room. The paint became thick as I rolled it. I am a professional painter and should have known better but really wanted to wrap up the project and not wait until the next day.
I hoped that the high dollar paint would settle down and even out when dry. It never did and I am living with the results for now.
Eventually I will have to sand skim coat sand again then seal the walls before painting. I will wait at least 4 hours after it is dry to the touch before applying next coat.
What you have there is called "stipple". It's not bubbles you're seeing--there's no air involved. It's just the pulling of the paint film into little mounds due to surface tension caused by the roller's "nap"--the absorbent material on the outside. Heavier nap clumps up somewhat, which results in it pulling away from the wall in a patterned manner.
A 3/8" nap roller is designed for textured walls--it holds more paint and presses it into deformities better than a smoother roller. I suggest you try a 1/4" nap or foam roller designed specifically for smooth surfaces. Also be careful to not work the paint too long. Once it starts drying it'll begin to pull into a stipple.
Finally, some paints don't self-level very well. Look for brands that claim to be very good at that--they'll fill tiny imperfections in your wall and level out after application to leave a smoother surface.