Zinsser BIN where did I go wrong?

[MonsterMunchies8]

I sealed a new mdf fitted wardrobe with 2 coats of Zinsser BIN. There are many videos on youtube showing you how to do this, so I think it was ok. It says on the tin that any paint can be used over it, so to get a solid white base I undercoated it with Dulux Trade spirit based undercoat. When it came to top-coating I used cream waterbased eggshell & it immediately 'separated' into thin brushstroke lines. It wasn't sticking. Why not? What do I need to do?

Well, your problem is not with the Zinser BIN; BIN accepts a water based quite well.
Sadly, you didn't need an undercoat, and it's the undercoat giving you problems.
You could lightly sand it, wipe down with Methylated spirit, and try again.
But you might have to move to an oil based top coat.
You could try ringing Dulux tech helpline ( all paint companies have one) - but ime Dulux are not the most helpful- standard reply is, you've done it wrong.

Or, if you've got enough Bin left, you could recoat with that, then use your original water based on top.
(Can you tell I'm still thinking as I'm typing?)
Is there somewhere out of sight, eg inside a door, where you can just try a small patch first, and see if BIN + water based does the job?

The trouble with that is that the Zinnser leaves the mdf too brown to go straight to topcoat. Looking at it it would take at least 3 top coats if not 4 to totally cover the brown mdf

Brown Zinsser, hmm - I'm guessing you mean there was too much brown of the MDF showing through, so you felt you needed an undercoat?
That's not a problem I've encountered.

And you've got an undercoat on now, so presubably the brown is gone?
Cause Zinsser itself should not be brown.I've covered BIN well with one topcoat of white water based silk.
What kind of undercoat and topcoat have you used- type and make?

Yeah not brown Zinsser but too much brown showing through. The Zinsser is white but despite continual stirring during use it gave poor opacity. Then Dulux trade undercoat, followed by Dulux waterbased eggshell.

Okay, if the brown is now covered,
I'd try some test patches, inside a door.
Current paint

1. + Zinsser + eggshell
2. another coat of the eggshell, see if that adheres/ covers better

it's probably not the zinzer but more the mix of spirit base undercoat and water based topcoat, you shouldn't mix the 2. I'd give it a light sanding and recoat with spirit based topcoat

It is not the BIN it is the oil based undercoat and a water based top coat. The undercoat is fine on the BIN, it is your top coat not adhering to your undercoat.

I never use oil based anything anymore. Always use BIN (woodwork) or BIN123 (walls) then water based on top. My woodwork as been super brilliant white for years using BIN and Valspar premium water based satin paint.

I know you said you stirred the BIN but both BIN and BIN 123 need a lot of stirring in the first place, probably more than you think.

In a test area can you keep going at it with the top coat to see if it comes together? I saw this on a video where someone was upcycling a piece of furniture and as she rollered it separated out, she said keep going and it will come together which I was surprised to see.

Agree with what’s been said the BIN is your primer/undercoat you need to stir really well, use 2 coats if necessary allowing time to dry between coats, then put your top coat on, probably 2 coats.
You should never mix oil and water products, it’s the oil based undercoat that is your problem, you didn’t need it.
Best thing to do not is lightly sand back and us an oil based topcoat, or sand off everything and start again without the oil based undercoat.

I used zinsser on a very dark wood mdf. Needed two coats of zinsser and then two coats of water based paint and the coverage was fine (was doing white). After two coats of zinsser it still looked really dark but two top coats were fine.

video shows you the separating I mentioned upthread but you can see how well BIN covers a dark brown panel from Ikea (laminated surface) on the first coat. It is all about the stirring incredibly well.

I agree with others, try a light sand test patch and re-coat.