Painting MDF

[Dave Plowman (News)]

Primed some MDF with Wick's wood primer which appeared to cover normally and left it a day to dry. Colour coat is a B&Q quick dry satin said to be suitable for wood and metal and not to need an undercoat - it's water based. Got a reaction - it formed a series of blobs with the primer not being covered at all between them. I'd used that combination elsewhere on MDF - but not from the same sheet - with success. And on bare whitewood.

A second coat after leaving plenty time for the first to dry produced the same results. I've now rubbed it down and used some undercoat - but will have to wait for that to dry before trying the topcoat again.

Any theories?

[Phil L]

the reaction you describe is known as 'cessing' in the trade - it's usually seen when paint is applied to an area contaminated with grease, even gloss paint cesses over grease.

You could try a light rub down with fine sandpaper prior to applying the topcoat

[Dave Plowman (News)]

The MDF was fresh from Wicks. Could the entire board be contaminated? But the primer went on evenly and dried quickly to a matt finish.

It will have to wait till Monday now. Working tomorrow.

[A.Lee]

I'd be more inclined to think it was a reaction between the 2 types of paint. I get it semi-regularly at work when emulsioning. The paint goes on well, then you either go over a piece again with the roller, and it pulls a strip of new and old paint off, or it starts bubbling. The only cure it is either sand down to bare plaster, or coat it all first in oil based primer. In your case, it may well be the water based top coat that is the problem. Either try a better quality paint, or get some oil based. Alan.

[Rod]

What sort of primer - oil or water?

If oil, I'd suspect that the very absorbent nature of the MDF has drawn oils and any volatile compounds well into the MDF. So the oils will not have dried and some volatiles may still be held in there below the surface.

If that is the case, warmth and time should work.

If water, water trapped below the surface could have caused problems. But we have heard a few times here that even water-based paints actually do contain solvents.

Again, warmth and time.

(This is just my guess - I am no expert.)

[Dave Plowman (News)]

That would be fine - except that I'd used the same combination elsewhere without problems.

[Dave Plowman (News)]

Oil based.

The primer appeared perfectly dry and as smooth as I'd expect. Ie, no reaction.

Don't think there was water present. Or at least no more than usual.

Talking about it at work I was told the clue is that the older MDF I'd used without problems had lost its 'solvents' or whatever - whereas the stuff I'd had the trouble with was new.