Advice on MDF finishing please

[davethesax]

I am making some toys, dolls houses etc. using 3mm MDF purchased from B&Q and unable to get a good finish. I have tried sanding with 240 paper and without sanding, used Rustins quick drying MDF sealer but I get a un-uniformed rippled effect also little particles of the boards structure grinning through the finish emulsion paintwork even with sanding between coats -rather like the grain rising in normal wood-.
Are there different qualities of MDF and if so how is rated and where can I get some?
Regards to all, David

 

[mailee]

Hello Dave, It is difficult to get a good finish on standard MDF. Really you need to use the Moisure Resisitant MDF which is a little denser and takes paint much better. That said it is possible to get good results with the standard stuff but takes more work. You need to seal the surface/edges first using A shellac based product or some thing that will fill the fibres. most of the pro's use an Acid cat lacquer either water based or solvent. these are quite expensive though as they come in 5 Ltrs. some people use watered down PVA glue but I have found this pretty difficult to sand. I have recently been recommended to use a primer called Zinnser Bin which is a shellac based product but I haven't tried it yet. Both the AC primers and the Shellac based sealer's are easy to sand with 320 grit paper and give a smooth finish ready for the top coats. HTH. :wink:

 

[pip1954]

i have found the same with thinner mdf you can get better but it's a bit like luck of the draw. when you buy the mdf you should feel the surface you should learn to know when it is rough and when it's not.
but 3mm is from my use of it seems to thin to get a good finish.
pip

 

[davethesax]

Thanks a lot mailee and pip for your advice. I managed to track down several suppliers of Acid cat and Zinnser Bin but only in what would for me be too large quantities.
May have to find something other than MDF.
Best regards, David

 

[Oryxdesign]

You can buy Zinzer in aerosol or 473ml. It's worth having around for covering moisture and other stains so it wont go to waste.

I don't think you can get 3mm mdf in MR

 

[twothumbs]

You should find the Shellac based BIN stuff at good paint merchants. It is quite thin and dries quickly. Useful to have as a very quick primer.

It comes in pint size equivalent tins.

 

[davethesax]

twothumbs and Simon. Thank you for your advice. Maybe I’m a bit thick but what is MR?
Regards, David

 

[Oryxdesign]

Moisture resistant, doesn't just apply to mdf (mrmdf) but could apply to plasterboard or chipboard and others, generally MR stuff is green or has a green core.