Walls and Ceilings - To smooth or not to smooth?

Our walls currently have large knockdown texture on wallboard, which is a pain to paint, especially with dark colors. We are considering smoothing out the texture (walls are currently painted with flat off-white), at least on some walls. It doesn't need to be smooth as glass, but maybe look more like the custom homes with flatter walls that look like spread on plaster. We're redoing a closet and thought about using that as our practice area, so it wouldn't show too much if we mess it up.

With that, here are our questions:
1. What would we use to smooth out the texture and do we have to sand the texture off the walls first or can we just slather away?
2. How do we estimate how much of the stuff we would need and how thick of a layer do we need?
3. Is this difficult to do (as in, is it more trouble to get a decent finish than to just paint several coats using a thick nap roller and hollering ARG! each time we find white speckles or thin areas peeking through the new paint months later)?

I should mention that we do like the look of the flat walls, at least in some of the rooms, so it isn't just our laziness in painting that we're considering this... but if it's difficult, we may rethink this whole thing. :confused:

Thanks for your patience in my long post!

From your local building supply store, purchase a box of regular drywall joint compound, a 5gal. bucket with a lid,(if you don't have a clean one), a plastic drywall pan and a 6" drywall knife.
Open the box, then the plastic bag and folding it down over the sides of the box. Empty the box into the bucket, (your aim will get better with practice),
then after removing the box from the bag, lift the bag by the edges, out of the bucket. Reach into the bag and lift by the bottom,(the mess is now back on the inside of the bag), reinsert the bag into the box, again folding it over the sides. This is a trash recepticle.
Trades people do this without getting their hands dirty.
From here you can thin, using water, the entire bucket or transfer some to the plastic pan and thin and apply from there with the 6" knife.
At the end of the day, tamp the bucket, scrape the material off the inside surfaces down into the bucket, smooth the surface and wipe the inside of the bucket with a damp sponge.
Add 8oz. of water as a float to the surface and place the lid on the bucket.

You could also purchase premixed 5gal. pails, which may have to be thinned for use.

Its also more expensive.

What you want to do is called skimming. Get some lightweight mud and apply it with a twelve inch drywall knife. You will also need a six inch knife and a pan. First coat of mud will fill in the knockdown. Second coat will fill in what you missed and third will make smooth. Sand at the end. Definitely good idea to start in the closet and practice.

Thanks Snoonyb and coops28! That doesn't look so bad after all (ok, well the process looks pretty easy anyway... the talent for getting is smooth has yet to be uncovered :) ) I'll just imagine I'm icing a really, really big cake.

Oh and Snoonyb... love the instructions... I would have dumped it everywhere and been covered like a pastry chef wallerin' in her vat o' flour on baking day!

We were all novices at one time.:)

Maybe I'm missing something - but I really can't work out why tradespeople buy their mud in boxes and then empty it into a bucket. At most you're saving a buck for every 5 gallons. For that princely sum you've wasted a few minutes and maybe managed to keep both yourself, the outside of the bucket and the floor clean. And first you need a bucket or two (which you could buy at the depot for $2.50 each!)
I'm halfway through my 2000sf basement and I now have half a dozen buckets or more which I use to store tools, keep clean water on hand (no running water down there), carry tools upstairs to clean up etc. That extra 6 bucks I've spent was SOOO worth it.
And I'll never need for a good strong bucket again.