In the archives this subject is pretty well covered. It seems an equal spilt between water and oil based primers. My inclination would be to opt for oil based. We all know MDF swells after water contact, so for that reason I'd not go for water based. This is to prime the bit of mdf that I've exposed by routing another profile over an old one, on some skirting that'll eventually get white gloss.
It does swell slightly, but shrinks again later.
So either works. Ive used emulsion, acrylic wood primer and sometimes oil base. I like the acrylic water based the best.
You MUST do at least two coats on the 'end grain'..sanding between coats.
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The MDF primer/sealer I've used was basically PVA. There is a tiny bit of raised "grain" but then you'd expect to rub it down prior to a finish coat anyway.
I've built loads of (kitchen) projects with MDF. I use this formula to paint them: One coat of water-based wood primer (cheap as possible - B&Q's is OK), then rub down with 120 grit, pay attention to the edges and routed features - maybe reprime if the rub down exposes too much. Undercoat appropriate for the top coat colour as recommended by the top coat manufacturer, rub down with 320 grit. First top coat, rub down with
400 grit with a hand block, then the final top coat. The result looks, and feels, like gloss melamine.If there are holes to cut, say for hinges, I cut them first, so that the primer gets into them too.
What matters is a complete film of water-proof paint over the whole piece, not what the solvent is.
R.
Rustin's do a specific MDF sealer, you can wipe it on, dries quick, I have used 2 coats then over painted on some MDF sills/boxing in the bathroom a year ago - works just fine.
For the best finidh ... I have also heard talk (from a couple or sources) of using a filler (eg. polyfilla), following routing a profile and then using the router bit to scrape the profile smooth - never tried it myself though.
HTH,
Alex.
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