Originally Posted by 235
I am starting to think about my MDF soffits and columns. How are the seams and screw holes handled if I decide to paint the MDF rather than cover with fabric. Any tips or suggestions on prep for painting would be appreciated.
For the seams, If you have the ability to do so I would suggest using a biscuit joiner, Kreg jig or doweling and a bit of glue to get the joints to line up as close as you can. The rest can be sanded for a smooth finish.
For the screw holes I would suggest predrilling AND using a countersink bit so you don't have any "mushrooming" of the material. A lot of folks like to use Bondo for the ultimate smoothness of finish, but Vinyl wood fill is also a good alternative. If you use Bondo, WEAR A MASK when you go to sand as the dust is not something you want to breathe.
For painting MDF, an oil-based
sealing primer (Such as Kilz, Zinsser, etc.). Do not use any water-based product on raw MDF otherwise you are inviting the possibility of the MDF swelling from the water uptake. Spraying the oil-based primer is the preferred option to maintain the smooth finish. Typical recommendation is to add 2 ounces of paint thinner to the primer for thinning before spraying so the material sprays properly. You will need to buy and use a special respirator mask (about $30) and make absolutely sure you have great ventilation. And be prepared for the room / house to STINK for the better part of the day. Drying times are quick at under an hour and two coats of primer with a light sanding between is recommended.
Once everything is sealed, paint as normal but I would also recommend spraying the paint if at all possible - two coats with a light sanding in-between. To thin a latex paint for spraying use a product called Flotrol at the same 2 ounce per gallon as a starting dilution. Any paint sprayer should have a small cup with a hole where you can time what it takes to go from full to empty to achieve the right viscosity. Paint sprayers vary, but they have instructions on how to achieve the right viscosity.
I just had to go through this entire process for painting the MDF / molding "wainscot" I did for our new son's nursery. I did a lot of research and asked a ton of questions from the paint professionals, so the synopsis above worked perfectly and I was astonished at the professional looking job at the end.
I hope this helps and good luck!