I've seen somewhere that using MDF will dull a bit faster than wood. If so how hard is MDF on the bits and should it be avoided.
it's not the hardness it's the abrasiveness...
IIRC there is about a 30 ~ 40% reduction in bit life...
economy bits don't last because of the softer carbide...
the dust is major detrimental to your health...
MDF is basically glorified particle board.
Just like particle board, MDF will soak up water and other liquids like a sponge and swell unless it’s very well sealed on all sides and edges with primer, paint, or another sealing product.
Because it consists of such fine particles, MDF doesn’t hold screws very well, and it’s very easy to strip the screw holes.
Because it’s so dense, MDF is very heavy. This can make it more difficult to work with, especially if you don’t have a helper who can help you lift and cut the large sheets.
MDF can’t be stained. Not only does it soak up stain like a sponge, but also because there’s no wood grain on MDF, it looks awful when it’s stained. (Kind of like staining particle board. Why would you bother?)
MDF contains VOCs (urea-formaldehyde). Off gassing can be greatly minimized (but probably not eliminated) if the MDF is encapsulated with primer, paint, etc., but care needs to be taken while cutting and sanding to avoid inhalation of the particles.