Sealing oak before concrete pour

How should I seal a piece of oak before pouring concrete up against it?

I've seen videos of people doing something similar, and they are using a spray of 'water based sealer'. What actually is it? Polyurethane?

Initial attempts have been problematic, because pouring concrete up against the end grain and keeping it damp for three days is obviously making it suck up dirty cement water. I tried a prototype with boiled linseed oil involved, but that didn't really seem to make a difference, possibly because I didn't give it much of a chance to polymerise before the pour.

I'm more than happy to put as much yacht varnish on the end grain as possible before the pour, but what about the actual tabletop surface? It's being cast upside-down, so a bit of runoff from the concrete is to be expected. Do I coat the wooden part of tabletop with poly before pouring the concrete, and if so, will I be able to get it off again? I'd like to give the tabletop the full linseed oil, shellac and poly treatment after I take it out of the mould, but I don't want to do this first because it may require some sanding to get stray concrete and silicone off. Basically I need a way of putting a waterproof seal on the oak that will sand off really easily, so another finish can be applied later. Is this mysterious spray sealer the solution?

Thanks very much for your help.

In all honesty, I can't think of any common use product that, applied just before a pour, would protect against being submerged in wet cement while it cures.

If I was attempting it, I think I'd grab some heavy plastic and tape to mask off the table surface though. At least as a 'first barrier.' Perhaps oil the surface first, without finishing, and leaving it to cure for a week or three before wrapping. Just so that any moisture that does get between the plastic and tabletop is less likely to penetrate and stain the wood.

As for sealing the edges and end-grain... I think I'd resort to something drastic there. After masking up the surface, I'd probably apply a coat of silicone all over the edges, wrapping over the top of the masking tape. This would mean you'ld have a perceptible gap between the slab and the concrete. How big would depend on how thick a coat of silicone was applied.

But you shouldn't expect the concrete to butt seamlessly against the slab anyway. Wood movement will eventually pull the timber away, giving you a visible crack between.

Hopefully someone else will make a better suggestion, but at least this may give you alternative methods to think about.

Concrete is basic and therefore incommodious to the growth of many organisms. If I were you I'd consider two treatments. Where appearance is an issue I'd use a water based silicone wood treatment with several coats. If appearance isn't important I'd use silicone first and a nice coat of liquid tar - roofing sealant. Polyurethane won't stand the pH of concrete very well.

Can you put in some sacrificial timber as form work the same dimensions as your oak and step it and fit the oak later?
that is how I have done it

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Originally Posted by samscement

How should I seal a piece of oak before pouring concrete up against it?

I've seen videos of people doing something similar, and they are using a spray of 'water based sealer'. What actually is it? Polyurethane?

Initial attempts have been problematic, because pouring concrete up against the end grain and keeping it damp for three days is obviously making it suck up dirty cement water. I tried a prototype with boiled linseed oil involved, but that didn't really seem to make a difference, possibly because I didn't give it much of a chance to polymerise before the pour.

I'm more than happy to put as much yacht varnish on the end grain as possible before the pour, but what about the actual tabletop surface? It's being cast upside-down, so a bit of runoff from the concrete is to be expected. Do I coat the wooden part of tabletop with poly before pouring the concrete, and if so, will I be able to get it off again? I'd like to give the tabletop the full linseed oil, shellac and poly treatment after I take it out of the mould, but I don't want to do this first because it may require some sanding to get stray concrete and silicone off. Basically I need a way of putting a waterproof seal on the oak that will sand off really easily, so another finish can be applied later. Is this mysterious spray sealer the solution?

Thanks very much for your help.

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Hi samscement

welcome to the forums
wow, for a 1st post that's not a bad one.

so what are you trying to achieve?
the *best* mould release agent -- especially when casting a concrete top upside down -- is a piece of 6 mm float glass.
handling a thin concrete top so that it doesn't crack as you turn it over can be a challenge.

If you are attempting to transfer the grain structure of the oak board to the concrete then you have a whole world of learning ahead of you.

a bit more info around what you are trying to achieve and how the concrete will be placed would assist.

There is a paste sealer which is designed for and intended for use to seal red oak.
What I would expect to need to do to seal an oak floor before any finish at all.

The concept is to smear it on then sand back the excess to the wood surface.
The pores, the vessel elements, are now permanently sealed.

I would not want to run anything up against the wood,
the trapped humidity will allow for fungal and bacterial damage
eventually.