Advice with terrain boards construction / warping

[Charlie_]

So, I've started work on my first terrain boards.

9mm MDF, supported with 20mm square battens underneath.

I'm planning to do both 60cm square ones, and 120cm by 60cm ones. The bigger ones will mean less gridlines on the table, and mean my roads and rivers won't have to be all stuck at right angles, and can have more natural curves to them.

So over the past few days I constructed 2 square ones, and one longer one. I'm not much of a DIY man, and went on a shopping spree to get the tools and equipment I needed. They have come out very nicely! The battens I have afixed with PVA glue and firmly screwed on.

So today I got them out to sand the edges down smooth.... I REALLY want them to line up smoothly with no big cracks. The two square ones seem perfect. Hurrah!

However the bigger one has warped along the long edges. Well, only on one of them significantly... The other ever so slightly, so that it probably won't be noticed.

Let me show you a diagram of how I have laid out the battens (1) (2), and to show you how the long one was warped (3).



As I said, the two square ones have worked fine. No warping. Now obviously I understand that the longer, rectnangular one will be more prone to warping.

Here's the question - would more / better designed battens help remedy the problem for the next ones I try?

Here's a diagram of two things I can try. On the first one (4), I have just added some more battens to the original design, which I could still add to the existing warped one. The other design (5) would give me a total of 4 length-ways battens, which I would hope would do a lot of good.



Do you good people have any thoughts on this? Any other batten configurations I should consider? Am I missing something obvious here?

The other potential problem is down to the PVA. Now I was thinking just the PVA along the edges where the battens are would no way be enough to warp the boards. Perhaps I underestimated PVA! Now here I definitely made a mistake, as I had actually bought some No More Nails for this exact purpose (I've been told it has less moisture so is more suitable for this?), but forgot about it! So on the next ones I will definitely use that rather than PVA. I also may well have put far too much PVA on. That could very well be the culprit right there....

I also used some black matt spray paint on the boards before applying the PVA, hoping that would seal them, but clearly it hasn't worked in this case (or the ammount of glue I put on overpowered it!).

There is also always the possibility that the board was already slightly warped before I started work! I should have checked it, and will definitely make sure to check the next ones.

I've read how applying PVA on the opposite side will counter-warp and help even it back out. I have done that just now, fingers crossed it works.

Another option is I could just give up on the 60x120 boards, and stick with 60cm square ones. Less warping to contend with, but will ultimately be less aesthetically pleasing when on the table.

So..... What tips do you have?
Do I need a better batten configuration on the large boards?
Is it just a case of over-doing the PVA?

[snitcythedog]

From your description it almost sounds like you had too much glue for the joints but I would not normally worry about that with 9mm MDF since that is thick enough in most cases to absorb the water from your glue.  The warping is a bit worrying since it does not bode well for adding the surface later.  I am wondering if the battening was warped to being with or if the original MDF sheet was.  Both situations would show up on the finished piece.  Next time you can try less glue and weighing the board down on a flat surface to stop warping to being with.  Gorilla glue is another option to replace the PVA since it carries less moisture vs coverage and foams up to fill any gaps.   
Your battening in option four would support the board but you really do not need quite that much.  I would just have a single cross member in the center of the board instead of two.   My preference since it would be less weight. 
Hope that helps and I am sure that others will chime in and give you some other options.
Snitchy sends.

[Charlie_]

From your description it almost sounds like you had too much glue for the joints but I would not normally worry about that with 9mm MDF since that is thick enough in most cases to absorb the water from your glue.  The warping is a bit worrying since it does not bode well for adding the surface later.  I am wondering if the battening was warped to being with or if the original MDF sheet was.  Both situations would show up on the finished piece.  Next time you can try less glue and weighing the board down on a flat surface to stop warping to being with.  Gorilla glue is another option to replace the PVA since it carries less moisture vs coverage and foams up to fill any gaps.   
Your battening in option four would support the board but you really do not need quite that much.  I would just have a single cross member in the center of the board instead of two.   My preference since it would be less weight. 
Hope that helps and I am sure that others will chime in and give you some other options.
Snitchy sends.

The wood for the battens was somewhat bowed in a few cases. I had thought that the MDF would have straightened the batten (it's just 20mm square pine) rather than the batten bowing the board, but perhaps I was wrong?

Perhaps I should have been more picky with the wood I chose for the battens. Annoyingly, I need it to be 20mm exactly.... as I've got 20mm craftfoam I will be using with layer boards to make rivers and such. B&Q is where I get everything here in the UK, and they've never let me down... except they don't have 20mm square stripwood. They have 19mm, 21mm, and every other dimension you could need, except 20mm square.
I found Homebase have it, but everything at Homebase seems a little cheaper and lower quality compared to B&Q. I had to sort through all their stripwood which was to find enough 20mm square ones, some of which were bowed. Perhaps I'll have a look for somewhere else local to get them....

As to adding the surface area later, I'm going to be using filler to texture it, rather than laying down PVA for sand, for what it's worth....

[SBRPearce]

Be sure to seal both sides of your boards before adding texture/glue/paint to the 'active' face - it's differential moisture absorption between the two faces that causes the moist face to swell, and the piece to warp.

To correct warping, I'd suggest trying to lightly moisten the cupped face and then weighting the piece down with as much weight as you can manage to stack onto it until it's dry.

Solvent-based adhesives like NoMoreNails, Liquid Nail and contact cement are good for avoiding this.

[Silent Invader]

My take: The battens are thinner than I'd have used and I'm not keen on a square profile.

I generally use 6mm MDF in 30cm squares without battens BUT introducing  too thin a batten that is not structurally graded (and therefore pretty much certain to take a turn in at least one direction) can warp the board as the batten warps.  That said, I wouldn't categorise 20mm strip wood as battening: IMO (but I could be wrong) as it's generally used in decorative situations and so lacks strength.

Where I have used battens on 30cm sq 6mm MDF I have used (approx) 35x25 PSE that is screwed and glued (narrower face to the batten, as that is more likely to warp than the wider face).

For adhesive, I always use Evostik Exterior Grade Resin (wood glue in the blue bottle) as it has low moisture content and provides for a strong, rigid joint.

One other thing to bear in mind is that the MDF might already have had a warp: the sheets are often stored on packing strips and might have developed the 'memory' of a bow. I generally only pick up MDF tiles when a new pallet has started, so that I can take a sheet from the top (it helps that I'm in B&Q several times a week).

Good luck.

 

Ps: Just a thought but if you add thicker battening (see para above) to the underneath as a plinth, that might well straighten everything out so that you can stick with the 20mm decorative edging on the top.

[OSHIROmodels]

The wood for the battens was somewhat bowed in a few cases. I had thought that the MDF would have straightened the batten (it's just 20mm square pine) rather than the batten bowing the board, but perhaps I was wrong?

Unfortunately, the pine will have warped the mdf if it was already bent.

You could try cross bracing underneath (mitred joists from internal corner to internal corner).

cheers

James

[SotF]

Actually, if you start with the one of a single middle cross, then put angle supports in them, it'll probably work even better, double them for two X shaped ones would likely work even more.

[Ssendam]

You could get your longer board using two 60x60 frames?

[Khadrin Stonetooth]

If the battens are warped they'll warp the MDF because of their own tension. I think your best solution is to purchase perfectly straight battens and replace the warped ones. Anything else will fail I'm afraid and your modules won't fit properly.

[Charlie_]

Thanks for the advice guys! Looks like a bent batten is the culprit

The next one I make I'll be much more careful with. Went out and got hold of some perfectly straight battens yesterday.

I've left the warped board under loads of weights, to see if I can straighten it out. You never know, it might work. If not, it could still actually be used..... As it's just one edge which is significantly warped, that one can just be relegated to being the outside edge of the table!

[Eric the Shed]

All my MDF 9mm game Boards are 50cm x 50cm square and are not battened - I treated both sides with PVA and then fixed GW Battlemat onto one side. All my terrain goes on top to create flexibility.




I did this over 12 years ago and no warping in sight. The advantage of not battening is you can use both sides of the board, storage is simpler, and they weigh less. When not being used they can stack on top of each other and the weight will keep any warping to a minimum

good luck forward going

[Peithetairos]

You could also try a first coat of Shellac before you glue anything on. That should be a good sealant against moisture.