Smooth walls to me is a must for a clean modern look. Most homes these days are either spray textured or skip troweled, but my preference has always been for smooth walls. The alternatives are cheaper and easier to accomplish, but I think smooth is a worth while investment.
I recently putty coated a few rooms in my house so I thought I would share some quick tips. This is definitely a skill that comes with practice, but it can still be achieved with less experience, you will just have to clean and sand a lot more to get the right effect.
First thing to do is get two types of mud if you are just going over existing textured walls, all purpose mud and topping. Use all purpose on the first and possibly second coat, and topping on the next coat (its lighter and easier to sand). Use two buckets for the job, one for water and one for the mud. Mix the mud fairly light, with consistency of pudding (mud comes pre-mixed but needs to be thinned for putty coating. Work an area, putting mud on a 5’x 5′ section or greater (some like to roll on the mud). Then come back over it with your knife to smooth it out, working from one end to the other, lifing the knife just slightly on one side to keep the lines to a minimum. You just want to leave a 1/16″ + layer of mud on the wall and the first coat is gonna have bumps. If your first coat is fairly smooth, you can switch to the topping for the 2nd and final, but if your new, you may have to sand between coats or possibly do a second coat of all purpose. Ultimately you can sand any mistakes out but make sure you put a good flood light on it before paint to ensure you have a majority of the imperfections cleaned up, as different lighting and day light will show problem areas.