Painting MDF

Wombat

Hi All.
We've had a new roof installed in a wooden conservatory and the builder has used MDF boards. I painted the boards with matt emulsion but this is now flaking off.
I tried re-rollering them with the same emulsion, but it just fetches the old coat off in patches. I tried painting a patch with MDF Primer and it has lifted the edges of the patchy emulsion up. What's the best way of repairing this paint job? Can I just keep sanding and priming on top of the old emulsion and then apply a top coat of some description, or is there a better way? TIA.
Steve.

Mickey666

Emulsion paint on wood is generally not a good idea but people are tempted because it requires no primer/undercoat.
Might be worth trying something like Bedec MSP - no primer needed and water-based so easy to clean brushes.

Wombat

Thanks, Mickey. I'm also considering fire retardant ceiling tiles glued over the painted MDF boards.

FreeBear

If the paint isn't sticking to the MDF, slapping glue over the top is not going to work. The strength of the bond is only as good as the weakest layer.

Jeepers_Creepers

Any chance of a photo of these MDF boards, Wombat? It seems a strange choice for a roof or ceiling - unless,possibly, these boards are coated in a melamine film. 

If so, that would also explain why the emulsion paint ain't sticking. If this was bare or primed MDF, emulsion paint would adhere with no issues at all. 

I suspect you now have the horrible job of having to remove the emulsion paint before trying something else. 

Grenage

MDF, conservatory, condensation, what can go wrong?

I'd probably have used an oil-based undercoat first, but suspect it's dampness that's making the paint come off.

Wombat

Jeepers_Creepers said:

Any chance of a photo of these MDF boards, Wombat? It seems a strange choice for a roof or ceiling - unless,possibly, these boards are coated in a melamine film. 

If so, that would also explain why the emulsion paint ain't sticking. If this was bare or primed MDF, emulsion paint would adhere with no issues at all. 

I suspect you now have the horrible job of having to remove the emulsion paint before trying something else. 

Hi Jeepers. I'm saying "MDF". It may not be - I'm no wood expert, but that's what I thought it was. It looks like wood shavings glued and pressed together.
These are some photos showing the boards with 1 coat of (peeling) matt emulsion, 2 coats emulsion (1st. coat comes off on roller in patches), 1 coat emulsion + sanding + 1 coat MDF sealer (edges of patchy emulsion lifting up).

Jeepers_Creepers

Ah. That's OSB or 'Sterling' board, and is what you say - wood shavings glued and pressed into sheets. 

It's good stuff - great of building, stable, strong. 

From what I recall from the last time I used this stuff, it does have a shiny and a not-shiny side, so i suspect the 'shiny' side has a coating which I guess is shrugging off your emulsion paint. (I last used this to build an internal store room at a Sea Cadet unit, and afterwards gave it a coat of emulsion - stuck like a sticky thing. but I don't know which side was painted!)

If the rest of the emulsion comes off easily, then you can consider repainting it in something more suitable - like Bedec MSP or Barn paint as Mickey says, or priming it first using BIN or similar. However, I don't think it's going to look all that good with the texture being very obvious, so I'd personally be inclined to over-board it using something like T&G cladding (light PVC stuff) or tiles as you suggest.

Need to check adherence first, tho' I can't see there should be any problems. 

Wombat

Jeepers_Creepers said:

Ah. That's OSB or 'Sterling' board, and is what you say - wood shavings glued and pressed into sheets. 

It's good stuff - great of building, stable, strong. 

From what I recall from the last time I used this stuff, it does have a shiny and a not-shiny side, so i suspect the 'shiny' side has a coating which I guess is shrugging off your emulsion paint. (I last used this to build an internal store room at a Sea Cadet unit, and afterwards gave it a coat of emulsion - stuck like a sticky thing. but I don't know which side was painted!)

If the rest of the emulsion comes off easily, then you can consider repainting it in something more suitable - like Bedec MSP or Barn paint as Mickey says, or priming it first using BIN or similar. However, I don't think it's going to look all that good with the texture being very obvious, so I'd personally be inclined to over-board it using something like T&G cladding (light PVC stuff) or tiles as you suggest.

Need to check adherence first, tho' I can't see there should be any problems. 

Thanks, Jeepers. I've sent for a pack of Polystyrene tiles just to try them out for adherence on one of the smaller boards.
It would make sense that the shiny side is inside - the other (outer) side is felted.
Thank you everyone for your comments.

Jeepers_Creepers

I'm sure you'll find something like StixAll will adhere to OSB with no issues, and lightweight tiles would only require a series of tiny dots spaced out evenly to hold them. The only issue is that the remaining emulsion is not stuck, so any adhesive could peel away taking the paint layer with it.

Sounds like you may have some wire-brushing or similar to do first :-(

(Goggles...mask...old clothes... :-)  )