Joining 25mm MDF edges, what to use?

[User1234567890]

I have a U/square shaped window sill/board that is 25mm thick and it will comprise of 3 pieces to make up the shape. I was wondering about the joints. As it is just a window board, would something like a 45 degree mitre and an adhesive like mitre bond be enough to stop it opening up when the seasons change, or will it need some kind of mechanical fastening as well?

I was thinking a cheap biscuit jointer, but wanted to ask if there are any other alternatives that can be used with 25mm thick MDF? Also, this will have to be assembled in its final resting place, rather than on the floor and lifted in, as it'll be large and quite heavy at 2.1m wide and 750mm for each of the shorter pieces.

What would you use in this situation? Thanks

 

[opps]

I personally would go for biscuits (or similar), either via a biscuit joiner or router.

I would use a decent quality PVA glue rather than mitre bond, and possibly dots of grab adhesive under it.

The biscuits will have the additional advantage of helping you to keep the surfaces flat.

I will however defer to @GhostIntMachine (who hopefully will be along soon)

 

[User1234567890]

Thanks mate, I've got some gorilla wood glue somewhere, should be OK with biscuits? And it is being held in place with pinkgrip. The thing now is which cheap biscuit jointer to go for. There's a couple of cheapish ones Amazon, an Einhell and a Draper, both of which I've not had much luck with in the past, at £60-70 and then it's £200+

 

[Scally-Ho]

If you are not going to be biscuit jointing on a weekly basis, what about a guaranteed refurbished machine?

https://www.screwfix.com/p/refurb-erbauer-ebj860-860w-electric-biscuit-jointer-220-240v/366px

OR my preference would be to rebate both sides of joint with a router and run a continuous thin 'biscuit' type joint set in a good quality adhesive; maybe a 2-pack adhesive.

 

[User1234567890]

Thanks mate, I will have a look at the jointer. I like the rebate idea, but I reckon it might be outside my level of competence, though cheers

 

[Scally-Ho]

Several biscuit joints along the jointing surface is almost as strong as a continuous one. Attack the job with confidence and that's half way there

 

[User1234567890]

Thanks again!. Will this Gorilla glue wood glue be the right thing for the biscuits or do I need to get something else?

 

[GhostIntMachine]

I've got some gorilla wood glue somewhere, should be OK with biscuits?

Absolutely the wrong glue! You need a water-based adhesive, something like EvoStik Resin W D3 (blue bottle). The reason is that biscuits are extremely dry and are made by compressing in a hydraulic press - so that when you add water based adhesive they expand to fill the grooves and lock the components together. PU glue isn't water based and doesn't do that. Leave any sanding for 24 to 36 hours in order that the glue sets fully and any moisture trapped in the MDF has a chance to dissipate. If you sand too early the biscuit shapes can telegraph through to the surface of the MDF and show when the surface is viewed in an obtuse light. This occurs because the biscuits swell then slightly shrink as the water content of the glue dissipates into the surrounding MDF material. A bit technical, but can sometimes be a real issue

Normally with biscuited joints you need to cramp the joint until it sets. Obviously that is impossible in your scenario - you may need to consider using some small screws through the joint until it sets which can be removed, 2-pack filled and sanded after the glue has cured. We sometimes need to resort to this on site. At the very least pull the joints together with masking tape - you want to avoid having to 2-pack fill the joints if at all possible.

Cheapest way to slot for biscuits is a biscuit slotting bit in a router - providing you have a router, that is.

Additionally, as this is a window board, and it isn't MR-MDF(?), I would strongly recommend that you seal the edges, back and bottom (i.e. the bits you can't see) before you install the board. An oil based primer, some old oil paint or something similar will do the trick. Window boards are often subject to high moisture/condensation and major variation in temperature throughout the day and the year and you want to avoid the MDF wicking-up moisture (and for that reason commercial window board is normally MR-MDF and comes pre-primed)

Because window boards are subject to swings in temperature and humidity adhesives like GripFil, Pink Grip, etc aren'talways reliable. We used to level the window boards front to back and end to end on packers, then drill through the top and (SDS drill, plug and) screw through the packs of packers to hold them in place (the countersunk holes were then either pelleted or filled, depending on the base material). In recent time s we have instead sometimes moved over to using the same sort of expanding foam you use on fire door casings (i.e. low expansion) to fix the boards. They still need to be packed-up level first, though, and may need to be weighted down until the foam goes off. Any discrepancies in plasterwork beneath the window board is made good after the window board is properly secured

 

[User1234567890]

Thanks for taking the time to write all that mate, it is much appreciated. Great info about the telegraphing thing, makes sense.

So this job has been about making this bay window, warmer, less draughty (drafty?) to avoid excess condensation. It was a crumbling mess under the old sill, so I have filled all the gaps and mortared it reasonably flat, with the hope that the pink grip would allow for a bit of adjustment. The problem is that I have some existing shutters that need to go back in, so I am working to the height of those. And to make things even more difficult, I’ve pulled the old ceiling out, insulated it and put new plasterboard on, so I’m left with about 16mm to play with, once it’s all level.

I was going to ask about screws through the joint. I have some tongue tite screws that I used to lay some flooring, they have a really small torx head on them. Do you reckon small pilot holes and then some of those along the joints? I’ve got a 5.5m piece of window board, so can experiment a bit.

Yes, the board is the pre-primed stuff. I have got a tin of waterproof black paint , I’d thought about sealing all the top of the brickwork that I’d mortared and the back of the board.

Thanks again

 

[GhostIntMachine]

I was going to ask about screws through the joint. I have some tongue tite screws that I used to lay some flooring, they have a really small torx head on them. Do you reckon small pilot holes and then some of those along the joints? I’ve got a 5.5m piece of window board, so can experiment a bit.

Yes, that's the sort of thing I do. In an ideal world you'd assemble it all on a bench with lots of nice sash cramps and cauls. Reality calls for a bit of extemporisation, so yes, I'd pilot and screw together. With MDF you need to be careful that your pilot holes are the right diameter and length and thoroughly cleaned out (and if you want to buy a bit of kit to pilot and countersink I'd highly recommend the Trend Snappy SNAP/CS/4 4mm drill/countersink for 4.2/4.5mm screws). If your pilot holes are too tight or too short you risk splitting the MDF, especially in end grain.

From what you are saying I'd definitely try a dry run before the final fit. As to Pink Grip being flexible - after it has cured it really isn't that flexible. You see solid gobs of it on the backs of old skirting which has been ripped out during refurbs and it's pretty solid after 10 years. but still pink!.. If you want a flexible construction adhesive rather than foaming-in you are better going to an MS polymer adhesive (sometimes called hybrid polymer adhesives) such as Soudall FixAll (first link I could find, but SFX and TS should both sell it or something similar). MS polymer adhesives are more expensive than standard construction grip adhesives (e.g. Pink Grip, GripFil, Gun-o-Preen, etc), but retain some flexibility once cured. I've used them on jobs like solid oak flooring for a long time

 

[User1234567890]

Thanks. I will have a look at the trend snappys. With the pink grip, I was just meaning some adjustability for levelling while it’s still wet, as it is quite thick and the board is quite heavy. But I am happy to look at other things. I just had the pink grip in already. The Soudal that you mentioned looks good, thanks

If the screws are coming back out of the joint when everything has dried, does it matter what type of screws, as long as they are coarse threaded and have the unthreaded bit for pulling tight? And obviously piloted correctly.

 

[Munroast]

Absolutely the wrong glue! You need a water-based adhesive, something

Gorrilla wood adhesive is just a white PVA, its fine with biscuits, they swell up like they do with any PVA
The polyurethane Gorrilla glue (the one they are famous for) is not for biscuits

 

[GhostIntMachine]

If the screws are coming back out of the joint when everything has dried, does it matter what type of screws, as long as they are coarse threaded and have the unthreaded bit for pulling tight? And obviously piloted correctly.

Not too much, no. As you correctly surmise you just need a deep thread.

 

[opps]

Gorrilla wood adhesive is just a white PVA, its fine with biscuits, they swell up like they do with any PVA
The polyurethane Gorrilla glue (the one they are famous for) is not for biscuits

It gets confusing when people talk about Gorilla glue. As far as I know, they are an American brand. I believe that the USA was late to the concept of PU glue. They seem to be willing to spend a lot on marketing though. My local Leyland SDM has the PU glue, the waterbased glue, the "super glue" and a MS polymer adhesive.

 

[Justin Passing]

I once spent half an hour at B&Q in the glue section, trying to work out what the different Gorrilla glues actually were. I think there were four different types! Visit website, or just avoid the stuffs and use something known?