I’m in the process of making a cabin bed for my son - I’ve constructed one supporting end, which is also a bookcase. I’ve worked with MDF quite a bit before, but I’ve never bothered to prime it before painting (mostly because I was constructing theatrical scenery, which didn’t have to last long anyway).
This time, I decided to do things properly, so I painted the bookcase with a thin coat of all-purpose primer - this is a solvent-based product that we’ve had unopened in the garage for a year or two. When dry, I rubbed it down a little and started applying a satin finish (a water based paint for multiple surfaces, including primed wood). It wouldn’t adhere properly to the primer and beaded up, leaving a sort of snakeskin pattern - the effect was worst where the primer was thickest.
Now, the satin paint is supposed to be suitable for painting over the top of gloss paint, if required (although it says you should rub down to key the surface first.
In the end, what I had to do was to take a cloth and rub a thin layer of the paint all over the primed surfaces, allow this to dry, then the paint seemed to go on and stay on.
There’s no way I’m going to prime the other parts, at least not with the primer I used this time, but what on earth could have gone wrong here?
Maybe mixing a solvent based primer and water based paint is a no-no? That’s the only thing I can think of.
That was my initial thought, but the water-based paint tin says you can paint it over existing gloss (even mentioning older paint surfaces that might contain lead, which would certainly have been oil-based).
I would suspect the solvent-based primer is the culprit (or maybe it hadn’t dried sufficiently yet, before applying the topcoat). For myself, I’ve always used KILZ or KILZ2 for my primer on MDF. Every topcoat I’ve ever used, oil or latex, gloss, flat or semi-gloss, has absolutely loved the stuff.
What was the solvent in the primer?
Like you, I’ve done many things with MDF other than painting it. I’m wondering if the high porosity of MDF is sucking in so much of the prime coat, that it needs to be primed twice? Most of the time, I’m cladding MDF with HPL, so a coat of contact adhesive seals the pores in one pass.
The primer had separated during the long sit on the shelf is my wag. And then it wasn’t remixed well enough. So you primed with an uneven mix of the primer. Where it was too much mat and notenough enough solvent, it acted like a crackling glue.
Note to everyone: Paint separates id left long enough. Sometimes it can’t be remixed either. Hit and miss as to what and how long from what I’ve seen.
I would’ve lacquer thinnered over it and sanded real well, then reprime with some new stuff. I’m afraid that your semi fix of a thin latex middle coat won’t last but will peel of or bubble. But then again, it might.
It may have been paint separation; the primer is a couple of years old; I mixed it thoroughly, but maybe not enough. I allowed about 36 hours for drying, which was way more than the tin said.
Interestingly, the top coat itself, when painted onto bare MDF, sticks like snot to a blanket - I tested it on an offcut and it looks like I’ll get better results from this than with a primer - I don’t think I’ll even need an extra top coat.