Here's a link that might be helpful...
http://www.taunton.com/finewoodworking/SkillsAndTechniques/SkillsAndTechniquesArticle.aspx?id=26508
The edge treatment is the key to painting MDF otherwise it will soak up coat-after-coat without sufficient 'build'. This applies to any sawn or moulded edges. One of best solutions for sealing MDF edges (on a small scale) is to use car body filler such as Isopon. Mix it up, smear it on well and cut it back with a suitable sand paper. Start with 150 and then go to 180 grit which should be fine enough for most paint finishes.
Most MDF these days has a polished surface when it leaves the factory. Acrylic should not sink into the surface too much. Two coats should be enough. Avoid waterborne finishes for the reasons stated in the article.
If you want to have your dolls house properly finished then have it sprayed with a polyurethane base and topcoat. A very durable finish (usually used for contract furniture and kitchens) which will resist scrapes and dings very well. It has to be applied professionally though using a spray-gun.
BEWARE! Polyurethane can contain a catalytic hardener similar to that found in Superglue (Isocyanate) which is hazardous if inhaled! For that reason it must be sprayed professionally in a correctly ventilated space.