Sealing MDF with 50:50 PVA/water mix?

[bobfog]

Just wondering if anyone does this old trick or have modern dedicated MDF primers negated the need? I've got some MDF that's currently drinking up the primer so I'm considering sealing it with a 50:50 PVA/water mix as per the title, then priming, then painting?

Thoughts or experience doing this appreciated!

Thanks
Bob.

 

[Stone Message]

Hi Bob,

Yes, I have always done just that prior to applying finish to MDF. I made the mistake of buying ready made sealer (too expensive then) but now do exactly what you have done. On end grain I have sealed it with polyurethane as it can drink up too much water based sealer.

Peter

 

[bobfog]

Thanks Peter. I'll crack in with this method!

 

[Stone Message]

Sorry, I forgot to say that the other advantage of the polyurethane on end grain is that when dry it can be sanded really smooth.

Peter

 

[bobfog]

Sorry, I forgot to say that the other advantage of the polyurethane on end grain is that when dry it can be sanded really smooth.

Peter

Would the polyurethane interfere with a waterbased paint going on top, do you think?

 

[windmill man]

Polyurethane and PVA (poly vynal acetate, if memory serves) are two compleatly different things with compleatly different properties !!!!.PVA is not the best thing to use on MDF if you are appling waterbased finish, it will rehydrate the PVA. Use a good quality Acrylic based primer sand and reapply if required .I have sucsessful applied  both water and solvent based finishes on it.

 

[Stone Message]

Polyurethane and PVA (poly vynal acetate, if memory serves) are two compleatly different things with compleatly different properties !!!!.PVA is not the best thing to use on MDF if you are appling waterbased finish, it will rehydrate the PVA. Use a good quality Acrylic based primer sand and reapply if required .I have sucsessful applied  both water and solvent based finishes on it.

That makes absolute sense to me - I will try it next time.

Cheers.

Peter

 

[bobfog]

Polyurethane and PVA (poly vynal acetate, if memory serves) are two compleatly different things with compleatly different properties !!!!.PVA is not the best thing to use on MDF if you are appling waterbased finish, it will rehydrate the PVA. Use a good quality Acrylic based primer sand and reapply if required .I have sucsessful applied  both water and solvent based finishes on it.

What if it's a waterproof PVA like Titebond III, does that change things?

 

[windmill man]

There really isnt a waterproof pva, its water resistant unless its been modified chemically

PVA is funny stuff and not always your friend

 

[junk731]

Old school a lot of the time can be bad as Wind has stated. If the MDF is soaking up that much its a sign of an extremely low grade porous product. With a 2lb shellac might be your best bet. I personally would be looking for a better MDF product that is designed to be more finish friendly. I know on this side of the pond there are several great products and a lot of crap.

John

 

[bobfog]

Old school a lot of the time can be bad as Wind has stated. If the MDF is soaking up that much its a sign of an extremely low grade porous product. With a 2lb shellac might be your best bet. I personally would be looking for a better MDF product that is designed to be more finish friendly. I know on this side of the pond there are several great products and a lot of crap.

John

It's top quality Medite Premier. The problem is I've laminated a few sheets together and put a significant roundover on the edge, so there's a large area of "end grain" where the slick face finish had been broken and is sucking up the primer.

 

[Holmz]

Around a sink one can use epoxy. It looks like a kindergarten project, but the sink going in over it.
I am not sure about in the middle of a sheet.something with a vacuum bag, or an initial layer to suck in might be the go?
If it does suck in and seal, then you have to worry about adhering to it.
The thinnest expos will go in the furthest... so a low viscosity and long pot life.
(This would be more of a science test)

 

[bkharman]

If it is getting painted, I use Zinnser BIN that has shellac in it. The stuff sticks to anything, and everything sticks to it it dries in 15 minutes, sands to powder and since it uses denatured alcohol, there is no swelling.

Cheap it is not, but it is a joy to prime with!

Cheers. Bryan.

Sent from my iPhone using Tapatalk

 

[greg mann]

CPES (clear penetrating epoxy sealer) is another option. It's pretty harsh on the nose but seals to beat the band and is easy to apply.

 

[junk731]

Old school a lot of the time can be bad as Wind has stated. If the MDF is soaking up that much its a sign of an extremely low grade porous product. With a 2lb shellac might be your best bet. I personally would be looking for a better MDF product that is designed to be more finish friendly. I know on this side of the pond there are several great products and a lot of crap.

John

It's top quality Medite Premier. The problem is I've laminated a few sheets together and put a significant roundover on the edge, so there's a large area of "end grain" where the slick face finish had been broken and is sucking up the primer.

The next trick with that much end showing is to make sure you sand to to 320P before applying any finish. The sanding with 320P gives a noticeable effect on grain grain raise and absorption. I don't know what paints are available where you are, thats why the recommendation of a heavy cut shellac.

John

 

[bobfog]

Old school a lot of the time can be bad as Wind has stated. If the MDF is soaking up that much its a sign of an extremely low grade porous product. With a 2lb shellac might be your best bet. I personally would be looking for a better MDF product that is designed to be more finish friendly. I know on this side of the pond there are several great products and a lot of crap.

John

It's top quality Medite Premier. The problem is I've laminated a few sheets together and put a significant roundover on the edge, so there's a large area of "end grain" where the slick face finish had been broken and is sucking up the primer.

The next trick with that much end showing is to make sure you sand to to 320P before applying any finish. The sanding with 320P gives a noticeable effect on grain grain raise and absorption. I don't know what paints are available where you are, thats why the recommendation of a heavy cut shellac.

John

Thanks. I've already sanded to 320, but I hadn't thought of BIN. I've got a little bit left over from a previous project, won't be enough to prime everything, but enough to seal those edges and just use a normal MDF primer on the rest of the surfaces and a second coat on the edges.

Thanks again.