My house is about 50 years old and many of the walls are plaster. If done nicely I would consider this a plus but it looks like my house was plastered/painted by a a 2nd grade art class. I don't have the money to get it professionally plastered so I ask; What is the best way to smooth them out? Can I sand them? Is it possible to cover them with drywall?
The best solution is to get them replastered. However, this is a messy and expensive job as you need to hack off the old plaster first. The expense comes from hiring a plasterer.
The cheapest solution is to cover them with drywall. This will give you a nice smooth wall.
You can glue the drywall directly to the plaster which will work if the current plaster is not too uneven. A better finish will be obtained if you batten the walls first ensuring that the battens are vertical and level horizontally. However, as @Tester101 points out you'll have to "pull out" all your sockets and light switches and have issues around door and window frames.
A better solution would be to rip the plaster off and then plasterboard, again either gluing or battening depending on the quality of the wall. Battening would allow you to add insulation and/or a void for feeding any cables you might want to add (ethernet, coaxial, power) in the future. This is still messy, but should be cheaper.
Sanding is possible, but it will generate a lot of dust and is a lot of effort for indeterminate results. If the plastering isn't too bad you might get a good finish, but you will have to skim the walls again to get a really smooth finish.
You asked for the best/cheapest way to get your plaster walls smooth, which is kind of like trying to get the most attractive girlfriend when you are the class nerd.
The best work is done by a professional, the cheapest work is done by a hack.
Now, which one do you want?
If it is just a light textural issue, you could hire a drywall guy to skim coat the walls with joint compound and sand it smooth. That would be pretty cheap.
However, without knowing more about your situation, there could be other issues that come into play that the repair person needs to know about, and address, for a good and long lasting finish.
For instance, if the walls are a glossy oil finish, you would need to prime them with an oil primer before skimming.
Or, if the paint is peeling, a good scraping, then oil priming would be in order.
If there are cracks or water damage, those are other issues.
If I were doing your work I would examine the wall for these and other issues, but basically addressing repairs, bonding and skimming are what is needed. Bucket mud joint compound can be used for skimming, but must be sanded without leaving any lines, which can be hard for a beginner with no instruction, plus it creates a lot of dust, unless you rent a dustless sander, which is also difficult for a beginner to use.
I would get a number of estimates from people who put themselves out as plaster repairers in your area, ask them what they are going to do to fix your walls. You will eventually know who knows what they are talking about and who does not. And you will have learned about what issues need to be addressed with your walls.
You may decide to hire the guy who knows his stuff. Get an estimate. If the estimate is more than you can afford, don't be shy to make a counter offer, but sweeten the deal...for instance, be very flexible with scheduling or offer a skill or good in partial trade. Try to find out what the other guy needs, especially if you have a valuable skill and negotiate.
Many trades are willing to do discounted work if it fills in their schedule or you can offer your skills/goods in trade for part of the payment.
Our house has majority plaster walls that were all covered with wallpaper. The process of removing the wall paper, some of the plaster cracked off (sometimes an area 3ft x 4 ft)
We used regular everyday drywall plaster; we used drydex; we used a mastic plaster to repair plaster walls.
Sanding the old plaster walls doesn't work. Its to dense and was just a waste of time. We skim coated over as best as we could; primed and painted until we could remodel that room.
Removing the old plaster / latte walls is just labor (and ALOT Of trash) - a room we're doing right now, no more than 11ft x 12ft produced 50 bags of trash.
Drywall is relatively inexpensive per sheet. Its easy to cut and with the help of a friend, easy to hang.
Opening the walls like this has a tendency to present more problems (poor insulation, poor electrical) but these are opportunities to increase the value of your house. Budget pending, updated electrical / voice / data / insulation will only increase the value of a home.
In our room, we decided to do it all; new doors / windows / etc... How far do you want to take it to have nice finished walls?
(the previous home owner went over plaster walls with drywall in the kitchen. This is a bad idea and only upset me more as it exposed a mildew / water leak problem)
Here is a affordable and less messy approach - In 2014 we remodeled our now clothing Boutique plaster walls with Burlap....Burlap is the "in thing"!
There is a huge selection of burlap color and designs available on-line. Burlap comes in various design, color patterns etc with widths say 5' wide and 50' lengths..cut as need and use burlap strips in seam sections. Before applying burlap We skimmed over the cracked and pealing plaster wall portions with sheet rock mud smoothing / sanding them ready for one good primer application....two' always better! It is necessary to prime after mud in order for double faced adhesive 3M tape to firmly adhere to wall...this attaches burlap as you slightly stretch and stick. The more vertical double faced strip tape eliminated ripples, tacking the upper and lower sections with a compressed staple gun also helps pull out wrinkles...we ironed the burlap before hanging to eliminate fold marks from mfg packaging.
Well, in fact you can plaster over them and get them smooth. It's not the easiest job in the world, but likely will give a far more appropriate final look compared to any drywall option.
First check for lead paint.
Second get a lot of dust control in place.
Third grind off the high spots. Aggressively grind any area with failing paint (this is why you need the lead paint test). Now repair and paint any trim or windows. Use a plaster repair kit to bind any loose areas back to the lathe.
Fourth, apply drywall mud. Here hiring a professional is likely your cheapest option: they can do it better.
Fifth, with a wet rag smooth out the various edges and add texture. You won't get a AWCI Level 5 finish, so don't try. Instead go for a light version of the streaks and bumps your existing wall already has.