Bizarre paint problem: Primer repelling topcoat

[Vortex]

Hello,

I've just built some shelving using MDF and have just encountered some kind of paint incompatibility the like of which I have never seen before.

Specifically I primed the shelving with Homebase own brand wood and metal primer (oil based)....but when I try to apply a top coat the paint has been repelled as if I am trying to paint wax!! On this occasion the top coat is Farrow and Ball "Modern Emulsion"...which is water based and 'kin expensive.

Pic here:

really puzzles me is that the same paint combination on bare wood just works with no problem at all.

What the heck is going on?

David

[Lobster]

I had just the same problem a few years ago when using Leyland acrylic (ie water-based) gloss over an oil-based primer. Also on MDF, but it was just the same problem where the substrate was timber.

I posted a similar "WTF?" plea for help here too, which you might be able to locate(!) IIRC the advice was not to mix'n match oil- and water-based paints, something which I've always made a point of adhering to since then, and have not had a problem again.

Still don't understand why it should happen though, if the underlying coat is fully dry.

David

[Vortex]

The primer was applied 24 hours ago. BUT dried in a chilly garage overnight, so not completely dry.

I'm just going to have to wait for the emulsion to dry and see how well it has adhered. Have a hunch I'll have to sand it all down and start again.

I'm so disillusioned I think I'll go to the pub.

D

[Maria]

Is MDF coated with anything? When I worked in a car bodyshop, we used to get problems sometimes with silicones in the old paint causing the new paint to pucker.

[Stuart Noble]

I'd guess the problem is simply that the oil based primer wasn't fully dry. If you've put a water based paint over it, that may well dry over the top and slow the solvent evaporation from the undercoat. I'd put the shelving indoors for 24 hours, then sand quite aggressively with 180 grit, and try the F&B coat again. The problem is likely to be more pronounced on MDF because the faces aren't as porous as wood and the undercoat is sitting on the surface.

[The Natural Philosopher]

Use farrow and ball oil based paint. Same colors.

Next time, don't prime. Just use emulsion.

[teddysnips]

Quite - old decorator's trick, if you've run out of wood primer (emulsion makes a crap primer on metal, though)

Edward