Thread: Bowing bench-top

Top and bottom surfaces are different.
Bottom has 6mm ply. Top and sides are sealed, but bottom is not.

You learn more by mistakes than when everything goes well.

The golden rule of finishing is "Always do the same thing to both sides."

I don't quite understand how you have constructed the benchtop.

100x20 hardwood laminated together to make a panel ~1500x800x20?

then you have applied a pine edging ~10x40 around the perimeter?

And then you glued all of that to a piece of 6mm plywood? <- this is the problem, if the construction method as I list it above was correct. But if you have laminated an unstable solid top to a stable substrate such as plywood, it will often give major issues.

As the solid top expands and contracts given the constantly changing moisture content of the timber (sealed or not, it makes little difference in this scenario), the surface not attached to the ply expands or contracts as per the normal, but the surface which is glued to the ply can't move. So the benchtop bows until the joints open up with unsightly cracks.

^^ Not quite how I've built it but I understand what your saying that makes sense.
Being recycled hardwood with very little moisture content, if I seal the top and bottom of the hardwood completely with modified oil polyurethane, than technically I'd be creating a moisture barrier to eliminate any timber movement at all???

You'll never completely get rid of timber movement, so don't bother trying.

However you do need to coat the bottom the same as the top and remove your substrate.

Coating both sides equally means that both top and bottom have equal sealing and will expand and contract relatively equally and shouldn't noticeably cup or bow.

It depends on what you coat the project with, most finishes still allow moisture to permeate through the finish (albeit at a much slower rate than unfinished).
Poly on the other hand doesn't so that doesn't help your cause. Wouldn't have thought the different substrate would have made a huge difference especially if you coated the underside with poly also.

Things I would ask yourself are:
- Did you let the wood acclimatise to your area? ie when you bought the wood home did you leave it in the shed for a few weeks? If the lumber has been sitting outside it would have absorbed alot of moisture. Bringing it home and dressing it before its had time to acclimatise would be asking for trouble
- Orientation of boards did you alternate the grain pattern on the boards?
- Check the surface of your work bench, is it bowing/sagging? did you support your bench across its length?
- Did you put stickers under the bench when it was sitting on the bench? This will allow air to flow under and above the board preventing it from unevenly drying/absorbing moisture

Hope that helps good luck with the bench

^^ Thanks for the help, I think because I didn't coat the underside of the hardwood with the poly it bowed heavily to the only coated side. It was straight before I started the poly coats, I thought because I covered the bottom of the bench with marine ply I could get away without coating the underside, little did I know.

All the feedback is much appreciated!

Welcome to the forum.